The United States voiced skepticism Thursday over reports from the Nigerian military that Boko Haram leader Abubakar Shekau had been killed in recent clashes with troops.

“The Nigerians have announced several times that the head of Boko Haram was dead and every single time we find out that it is not true,” a senior State Department official told reporters.

“What I read recently is that Shekau’s lookalike or some Shekau impostor was killed and then I read later that maybe Shekau himself was killed. I don’t put a lot of weight on those stories until we see more evidence,” the official added, asking not to be identified.

Shekau: is he alive or dead? Americans want more evidence

The Nigerian defense ministry said on Wednesday that Shekau was dead and that one Bashir Mohammed, killed during recent clashes with troops, was a lookalike who had been impersonating him in videos.

The announcement was the first time that the military high command had said Shekau was dead, after refusing to confirm two previous claims from police and a regional task force in 2009 and 2013.

But analysts also dismissed it as propaganda, and the State Department official highlighted that Boko Haram was still carrying out attacks.

The United States remains committed to trying to find some 200 schoolgirls kidnapped by the militant group in April, but the official acknowledged it was “a huge challenge.”

“Many of them were taken into the forest and given the time that is passed it is possible that many of them have been taken to other locations,” the official said.