WASHINGTON (Reuters) - An inert U.S. Hellfire missile sent to Europe for a training exercise ended up by mistake in Cuba in 2014, in a serious loss of military technology, the Wall Street Journal reported on Thursday, citing people familiar with the matter.

U.S. investigators are trying to find out what went wrong and authorities have been trying to get the Cuban government to return the air-to-ground missile, which is often fired from helicopters and used as an anti-tank weapon, those familiar with the matter said, according to the Journal.