How could the USS Maine explosion have been investigated?

It gets re-investigated periodically. An article here discusses some of the more recent times:

After the war ended, two more investigations were conducted in 1898 (one by the Spanish, and the other by the U.S. Navy), both reaching different conclusions. While the Navy insisted the Spanish were to blame, Spain’s examination of the evidence held that the explosion was clearly an accident caused by the crew.

Another inquiry by the U.S. government took place in 1911 in conjunction with efforts to recover the bodies of many of the Maine’s crewmembers who remained entombed in the wreck. This investigation also blamed sabotage. A fourth analysis in 1974 was led by Navy Adm. Hyman Rickover, followed by a fifth in 1998 by the National Geographic Society. The most recent research was the subject of a 2002 television show called “Unsolved History.” Despite the numerous studies, however, none of the investigations were able to conclusively state what caused the blast...

Question: After the explosion of the USS Maine in 1898, is it possible if the US asks Spain if they actually placed a mine?

At the time no. The United States Navy which was the primary investigative body, was not an impartial organization. Their initial investigation did not affix blame for the explosion on Spain but rather absolved themselves from responsibility. The US Navy having been warned of the danger of spontaneous coal explosions just months before the USS Maine catastrophe, was mostly concerned with proving the explosion in the magazine was caused by an external ignition source. Thus absolving themselves from blame in the death of 266 US sailors. The US Navy pre-war investigation in 1898's only conclusion was the explosion source was external to the ship. It did not place the blame on Spain, rather influencial US newspapers did that.

The US public was primed for war by two newspaper barons which were involved in a circulation war and were using the Spanish-Cuban troubles to sell papers and incite public opinion against Spain.

The US Navy did approach Spain many years later(1970s) to incorporate the Spanish investigations with those of the US to revise the explanation for the explosion which was reported in 1898 and 1911.

Facts:

• 1895 - Cuban exile José Martí, launches a 3 pronged invasion of Cuba seeking Cuban independence from Spain.

• October 1897 - The Spanish Prime Minister Antonio Canovas del Castillo, a long time supporter of Spain's ownership of Cuba was assassinated by the Italian anarchist Michele Angiolillo. Destabilizing Spain's government.

• In response to a revolt in Cuba, Spain cracks down brutally in Cuba. Spurred on by US Newspaper, and the accompanying public opinion the US dispatches the USS Maine to Havana as a way of announcing both the US eye was on Cuba and the US desire to play a role in events there.

• January 27, 1898 - An investigative board, warned the Secretary of the Navy about spontaneous coal fires that could detonate munition magazines on US warships, many of which situated such munition magazines next to coal storage bins.

• February 15, 1898 - USS Maine exploded and sank killing 266 of 355 sailors.

• March 21, 1898 - A USN board of inquiry authorized by the McKinley Administration concludes unanimously that the destruction of the USS Maine was caused by an outside explosion but declines to identify the source of that explosion. They make note to two facts.

  1. There had never been a case of spontaneous combustion of coal on board the MAINE.
  2. There was a distinctive V shaped hull damage to the ship which indicated to the board that an explosion source was under the keel external to the ship.

• April 21, 1898. - The United States Declares war on Spain

• August 13, 1898 - The War ends end with the Spanish Pacific and Atlantic fleets destroyed and many of Spain's colonial possessions in the hands of the United States.

Short Answer:

No the atmosphere of blame at the time did not permit collaboration with Spain. Yes that collaboration did eventually occur but only many decades after the incident and resulting war.

Due to the extremely hostile public opinion of Spain in the United States in 1898, manufactured by yellow journalism and newspaper circulation wars; The sinking of the USS Maine brought on what many Americans believed was an inevitable and desired war with Spain. The US Navy's inquiries of 1898 and 1911 were mostly concerned with shifting blame for the explosion and associated deaths away from the US Navy. Although the 1898 Navy Board of Inquiry did not name Spain as the source of the explosion, in declaring the cause of the explosion as external to the ship(USS Maine), the already agitated US public was left with Spanish hostilities as the next most plausible explanation for the explosion.

No in that atmosphere Spain was not seen as an honest broker. Spanish denials were ignored, and Spain's own investigation which pointed to internal explosion of a black powder magazine due to coal fire was dismissed at the time. It was not until many years later that Spain was consulted in a meaningful way.

In 1974 Admiral Hyman G. Rickover asked naval historians to compile a complete record of the investigations to date, including Spanish investigations and to seek modern expert opinions as to the source of the explosion. Modern experts reviewing all the data including pictures taken of the explosion and the excavated hull from the 1911 investigation as well as incident reports of numerous coal fires in US, British, and French contemporary ships; concluded that the explosion was "without a doubt" started internally.

Subsequent investigations point to the type of coal which the USS Maine had taken on(bituminous vs anthracite), the likely moisture levels of the coal bins after months in the Florida Keys and Cuba, and the proximity ( mere inches ) between a coal bin to the exploding magazine as the causal for the magazine explosion to be coal fire. No evidence of an external source for the explosion has was found.

Detailed Answer:

In the late 1890's two of the most influential newspapermen in the history of the United States were locked in a circulation battle. William Randolph Hearst who owned many newspapers including the New York Journal, and Joseph Pulitzer (Pulitzer Prize), who owned the rival New York World News; in order to outdo and outsell each other feed the public embellished or outright false stories of Spanish atrocities before and after the USS Maine explosion to sell papers.

Locked in a circulation battle, both papers embellished stories and sometimes made them up all together. The competition to outdo each other began when Hearst and Pulitzer saw an opportunity to sell more papers by focusing on the rising tensions between Spain and one of her Latin American colonies, Cuba.

As the struggle for Cuban independence intensified, Spain took brutally repressive measures to halt it. To protect U.S. citizens and property after anti-Spanish rioting in Havana, President William McKinley ordered the battleship USS Maine to Cuba.

Meanwhile, Hearst hired artist Frederic Remington to capture images of the war that was supposedly about to start. But when Remington arrived in Cuba he found that tensions had subsided and telegraphed Hearst to tell him there would be no war. The artist was flabbergasted when the newspaper magnate replied: “You furnish the pictures and I’ll furnish the war.”

After the Maine exploded in Havana harbor on February 15, 1898, both papers began publishing sensationalist accounts. Despite the fact that the cause of the explosion was unknown, the World ran a story about the ship being blown up by a Spanish torpedo along with a picture of a violent explosion.

Not to be outdone, the Journal ran a similar story, claiming it would give a $50,000 reward to anyone with information on the attack.

The 1898 unanimous US Navy board of inquiry which found the source of the explosion was external to the ship had glaring mistakes in it's findings.

US Law Library of Congress, August 4, 2009
Evaluating the 1898 Report. The board of inquiry did not make use of many technically qualified experts. George W. Melville, the Navy’s Chief Engineer, doubted that a mine caused the explosion but was not asked for his views. He suspected that the cause of the disaster was a magazine explosion.8 Philip R. Alger, the Navy’s leading ordnance expert, told the Washington Evening Star a few days after the blast that the damage appeared to come from a magazine explosion.9 Many ships, including the Maine, had coal bunkers located next to magazines that stored ammunition, gun shells, and gunpowder. Only a bulkhead separated the bunkers from the magazines. If the coal, by spontaneous combustion, overheated, the magazines were at risk of exploding. An investigative board on January 27, 1898, warned the Secretary of the Navy about spontaneous coal fires that could detonate nearby magazines.10

The Maine took on bituminous coal, which was more subject to spontaneous combustion than anthracite coal.11 Fresh surfaces of newly broken coal oxidize as part of a chemical reaction that produces heat. If not dissipated, the heat accelerates the reaction. A higher moisture content in the coal will increase the tendency to heat up. The ship had spent most of the last three months anchored at Key West, Florida or nearby. The tropical climate in that region and in Cuba would ensure that the coal was moist.12 Ships installed alarms in the coal bunker to detect overheating, but often fires from spontaneous combustion smoldered deep below the coal without raising the temperature near the alarm.13 Fires from coal bunkers were frequent occurrences. From 1894 to 1908, more than 20 coal bunker fires were reported on U.S. naval ships.

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US Law Library of Congress, August 4, 2009
Rickover Study.
In 1974, Admiral Hyman G. Rickover asked naval historians to take another look at the sinking of the Maine. He was particularly interested in the cause: was it an external mine or an internal explosion? A team of experts reviewed government records, archival sources, personal papers, contemporary newspapers and periodicals, and other sources. Rickover encouraged them to contact the Spanish naval attaché to see what materials were available from archives in Spain and to seek help from other countries, including France and Great Britain, to learn what their navies had experienced with ship explosions. The scholars turned to professional engineers to interpret photographs of the wreck and study the ship’s structure. Their study determined that the explosion was, “without a doubt,” internal.

Since the United States Navy's final investigation in 1974 there have been a few other reputable inquiries supporting the findings of the 1974 US Navy inquiry. A more comprehensive list/history of spontaneous coal fires in American ships (13) incidents in the 3 years leading up to the USS Maine explosion. A National Geographic sponsored investigation using computer simulations investigating the proximity of a coal storage unit with the black powder magazine which exploded. Turns out the magazine which exploded was inches / steel bulkhead away from a coal storage unit. This simulation showed a coal fire in the specific coal storage bin would have quickly obtained the heat necessary to ignite black powder across the steel bulk head which separated them.

Sources:
The United States Law Library of Congress
History News Network
Wikipedia: Spanish American War