A man who opened fire on a civilian police employee in the lobby of the Plano, TX, police station and was killed by officers Sunday may have been inspired by foreign Islamist terrorists.

Police say Imran Ali Rasheed killed a Lyft driver in Garland, TX, before he attacked the Plano police station.

Rasheed ordered a Lyft and fatally shot the driver, Isabella Lewis. Her stolen car was found a short time later outside the Plano police station. It appears he used the stolen car to drive to Plano and attack the station, CBS reports.

Matthew DeSarno, special agent in charge of the FBI’s Dallas office, told the Associated Press that investigators believe Rasheed acted alone but that the 33-year-old left a letter indicating he “may have been inspired by a foreign terrorist organization.” He did not offer more specifics on the letter and declined to identify the group.

DeSarno said Rasheed was the subject of a counterterrorism investigation from 2010 to 2013, when the case was closed after agents determined Rasheed did not currently pose a threat. The agency’s regional terrorism task force is working on the investigation of Lewis’ killing, he said.

Authorities said there was no known connection between Lewis and Rasheed and it’s unclear why he went to the Plano Police Department. “We have no idea why he came to Plano to find police officers,” said the city’s police chief Ed Drain.