U.S. officials are asking how a Hellfire missile used in a training mission in Europe has ended up in Cuba.

The Wall Street Journal reported Thursday that U.S. officials have asked the Cuban government to return the Hellfire missile. The report says it marks one of the most mysterious disappearances of sensitive U.S. military technology.

U.S. officials say the dummy missile was mistakenly shipped from Europe to Cuba in 2014. They say developer Lockheed Martin sent the missile to Spain. The NATO military used it in a training exercise. The missile was then to be returned to Florida. But, it disappeared after it was sent on a long journey through Europe.

By the time officials noticed the missile was missing, it was already traveling from Paris to Havana on an Air France flight.

The Wall Street Journal reports that U.S. officials are investigating whether the disappearance resulted from mistakes or whether spies or criminals were involved.

One U.S. official told the Associated Press that Lockheed Martin made a shipping error. The official said the United States is working with the company to return the missile.

The Hellfire missile did not contain explosives. But it did have sensors and targeting technology. U.S. officials say they fear the technology could be reconstructed. Officials are also concerned that Cuba would share the technology with other governments, such as North Korea.

The United States and Cuba ended diplomatic relations in 1961, but reopened relations on July 15, 2015. However, distrust remains between the two governments.

I'm Ashley Thompson.