ALEXANDRIA, Va. (AP) — For years, authorities had concerns about Metro Transit Police Officer Nicholas Young: He traveled to Libya and boasted of joining rebel groups there, and he even described his collection of Nazi memorabilia to law enforcement, according to court documents. But until last month, authorities said, he hadn't committed a crime.

Now Young, 36, of Fairfax, Virginia, is the first law-enforcement officer in the U.S. to be charged with a terror-related crime, after prosecutors say he bought about $250 worth of gift cards in an FBI sting for someone he thought was working with the Islamic State group.

Young was arrested Wednesday at Metro Transit Police headquarters in Washington and charged with a single count of attempting to provide material support to a terrorist group. According to an FBI affidavit, Young bought the gift cards last month intending that they be used by the Islamic State group to purchase mobile messaging apps. But the person he gave the cards to was actually an undercover FBI officer, the affidavit said.

He made a brief court appearance Wednesday afternoon, wearing a T-shirt and what appeared to be his uniform slacks.

David Smith, who was appointed to represent Young after the hearing, said he could not comment because he had not yet been able to research the details of the case. A status hearing was scheduled for Thursday afternoon.

If convicted, Young could face up to 20 years in prison.

Young had been under surveillance since 2010, and he traveled to Libya at least once in 2011, where he said he joined rebel forces seeking to oust dictator Moammar Gadhafi, the affidavit said. He traveled with body armor, a Kevlar helmet and other military-style items.

Young was deeply paranoid about law enforcement spying on him, often taking the battery out of his cellphone when he wanted to go somewhere and talk, the document said. Young frequently told one undercover source to be wary of potential informants, according to the affidavit.

On Jan. 24, 2011, an undercover officer said Young told the officer he once aimed an AK-47-style rifle out of a window at his home, scanning for law enforcement he believed was watching him. On another occasion, he grew angry that the FBI talked to his family and co-workers and said he wanted to find the FBI agent and kidnap and torture her.

The undercover officer said he "doubted that Young seriously intended to act upon those words," according to the affidavit.

Authorities were not the only ones who had concerns about Young. As police searched Young's townhome in Fairfax on Wednesday, neighbor Dina Ahmad described him as standoffish and said he had occasional run-ins with the homeowners' association over his cluttered front lawn.

He often worked on his car at late hours, and the car was adorned with anti-Israel bumper stickers, she said. Asked if she was surprised to learn of the charges against Young, she said no.

"We knew something was weird about him," Ahmad said. "You just kind of got that creepy vibe off of him."

Joshua Stueve, spokesman for the U.S. attorney for the Eastern District of Virginia, said Young posed no threat to the Metro system.

Metro spokesman Dan Stessel said Young, who had been employed since 2003, was fired.

"Obviously, the allegations in this case are profoundly disturbing. They're disturbing to me, and they're disturbing to everyone who wears the uniform," Metro General Manager Paul Wiedefeld said in a statement.

FBI spokesman Andrew Ames confirmed that Young is the first law enforcement officer to be charged under the federal government's terrorism law.

Young was an associate of two other people charged with terror-related crimes. In 2010, law enforcement interviewed Young because of his links to Zachary Chesser, who eventually pleaded guilty to trying to join the militant group al-Shabab and to issuing threats to the makers of the "South Park" cartoon series after they penned an episode he found insulting to Islam.

Young also met regularly with Amine El Khalifi, who pleaded guilty in a sting operation in which he planned to attempt a suicide bombing at the U.S. Capitol in 2012.

In his years under surveillance, Young frequently made alarming comments that did not rise to a criminal level. During one conversation with an undercover officer, Young said if he was ever betrayed by someone, "that person's head would be in a cinder block at the bottom of" a lake.

In March 2015, he raised suspicion when he brought a large amount of ammunition, AK-47s and a pistol to an off-duty weapons training event provided by another Metro officer. Young said he owned even more weapons, according to the affidavit.

In a June 2015 interview with law enforcement, he described dressing up at Halloween parties as a jihadist who had beheaded a hostage and as a Nazi. He told officers he collected Nazi memorabilia and he had a German eagle tattooed on his neck.

According to George Washington University's Program on Extremism, Young is the 100th person since March 2014 to be charged with an offense related to support for the Islamic State group.