Although the recruitment video has circulated among extremist groups for some days, intelligence analysts now believe the man with the blurred face is a 22-year-old from Florida who blew himself up last month in a suicide attack on Syrian government forces that killed 37, according to senior American government officials.

The man, Moner Mohammad Abusalha, who took his own life in a truck bombing mission, is one of roughly 100 Americans who have tried to travel to Syria to fight alongside Islamic extremists, or who have actually done so. American officials express deep concerns that the video may inspire others to follow his path.

The American authorities had tracked his indirect travels to Syria, but they knew very little about him at the time. It is not illegal to travel there, and many others have done so for humanitarian reasons. It was only after he arrived in Syria that the authorities here learned through intelligence sources that he was planning a suicide attack, senior American officials said.

Once Mr. Abusalha’s intentions were clear, there was little the United States could do to stop him because there are no American or allied forces in Syria, and certainly none who could have taken action inside the militant organization that Mr. Abusalha had joined, according to government officials. Had the authorities known before he arrived in Syria that he intended to fight alongside extremists, they most likely would have had enough evidence to charge him with providing material support to terrorists, as they have done with several other Americans.

The officials declined to say how the United States obtained intelligence that he was fighting alongside militants and was planning to blow himself up in a suicide truck-bomb attack. But in the past year, the authorities have obtained similar information in Syria from contacts on the ground, electronic intercepts like cellphones and foreign intelligence agencies.