The US suicide bomber who blew himself up at an army post in northwest Syria spent several months back home after training as a militant, The New York Times reported Wednesday.

The report highlights a concern from Western powers that Syria is becoming a training ground for citizens who could then return as battle-hardened veterans, ready to carry out attacks at home and abroad.

Moner Mohammad Abu Salha, alias Abu Hurayra al-Amriki, was believed to be the first American national to carry out such an attack in Syria’s three-year-old war.

The American jihadist on May 25 drove a truck bomb attack onto an army base in Jabal al-Arbaeen area of northwest Syria’s Idlib province. It is unknown how many were killed.

Citing previously unreleased information from a US investigation, the New York Times reported that Abu Salha trained with an extremist group in Syria, returned home for several months, and then left the country a final time.

An FBI spokesperson refused to comment directly on the investigation, but told the Times: “This incident exemplifies the challenges faced by the FBI in detecting US citizens who seek to travel to Syria to engage in jihad.”

Officials told the Times the US government was only able to determine Abu Salha’s intention of carrying out a suicide attack after he arrived in Syria the last time.

“He was on the path at that point and had decided on launching the attack,” one law enforcement official said. “It was fairly close in time that we learned that and that he launched it.”

Earlier, the FBI and Department of Homeland Security had some indication of the Florida man’s first trip to Syria, but had no information about his training as a militant or his intention of becoming a suicide attacker — thus giving authorities no reason to block his travel or detain him.

Estimates of the number of foreign fighters who have flooded into Syria in the past three years range from between 9,000 to 11,000.