An American from Florida launched a suicide bombing against Syrian government troops earlier this week.
It is believed to be the first time a US citizen has been involved in such attacks since the start of the Syrian civil war.
The man's name is Moner Mohammad Abu-Salha, State Department spokeswoman Jen Psaki said in a statement that provided no other details about him.
Abu-Salha was thought to be the first US citizen to be involved in a suicide bombing in Syria's three-year civil war, she said earlier.
Democrat Senator Bill Nelson told reporters in Miami that the American suicide bomber was from Florida.
At least 160,000 have died in the fighting between government forces and opposition forces seeking to oust Syrian President Bashar Assad.
Opposition forces had identified the man who carried out a May 25 truck bombing outside a restaurant in the government-held city of Idlib as Abu Hurayra al-Amriki and said he was a US citizen. The name al-Amriki means "the American".
It is unknown how many people died in the bombing. Opposition rebels with the al Qaida-linked insurgency al-Nusra Front said Abu-Salha's truck was laden with 16 tons of explosives to destroy the al-Fanar restaurant in Idlib, a gathering site for Syrian troops.
Asaad Kanjo, an opposition activist based in the town of Saraqeb in Idlib province, said he heard that Abu-Salha arrived in Syria a few months ago and tore up his American passport upon arrival.
Mr Kanjo said even a local commander with the Nusra Front was surprised about Abu-Salha, quoting him as saying that people do all they can to try to go to the US to get an American passport, and this man came here and got rid of his.
"From what I heard, I believe he was an American of Arab origin. People said that he spoke Arabic with a foreign accent, and he used to speak classical Arabic," Mr Kanjo said.
He added that Abu-Salha was a member of the Nusra Front but was not a commander: "Most probably he came to carry out this attack."
The truck bombing was one of four by suicide bombers who attacked over the course of a day in the area in Idlib province.