Updated: 4:43 p.m.

An Army soldier who had been granted conscientious objector status because of his Muslim faith purchased bombmaking materials and planned to attack military personnel outside Fort Hood, Tex, the same base where 13 people were killed by an Army major in a 2009 shooting , authorities said Thursday.

Pfc. Naser Abdo, 21, was arrested in the city of Killeen and was expected to be charged later Thursday with possession of bombmaking materials. Officials said they believed he was acting alone but that he presented a serious threat.

“I would classify this as a terror plot,” Killeen Police Chief Dennis Baldwin said.

An FBI spokesman said the materials found in Abdo’s motel room included gunpowder but declined to identify them further. A clerk at a gun store in Killeen told the Associated Press that Abdo arrived at the store earlier this week and bought six pounds of smokeless gunpowder, three boxes of shotgun ammunition and a magazine for a semiautomatic pistol. The clerk said that he alerted local authorities after Abdo indicated that he didn’t know much about the items.

The store was the same one where Maj. Nidal M. Hasan bought a high-powered semiautomatic pistol before officials said he went on a shooting rage at the military base almost two years ago.

“We would probably be here having a difference briefing if [Abdo] had not been stopped,” Baldwin said.

Abdo, of Garland, Tex., had been stationed with the 101st Airborne Division at Fort Campbell, Ky. In May, he was granted conscientious objector status after saying that his faith prevented him from military service. He was to be discharged by the Army, but the action was placed on hold after he was charged with possession of child pornography .

Abdo denied the allegations, and said that he believed the military was retaliating against him for trying to leave the service. Abdo went AWOL around July 4 weekend, authorities said.

James Branum, an Oklahoma City lawyer who represented Abdo in the earlier cases, said his client learned of the child pornography charges on the same day he was notified that his discharge had been granted.

Branum described Abdo as gracious and hospitable, and also “very devoutly religious.” Abdo’s mother lives in the United States, Branum said, but his father lives in the Middle East.

In a statement posted on Fort Hood’s Web site, the Army said that Abdo is in the custody of civilian authorities and that jurisdiction over any potential new charges has yet to be determined.

“If returned to military control, he may face additional charges, including AWOL,” the statement said.

Erik Vasys, a spokesman for the FBI’s San Antonio field office, said police in Killeen arrested Abdo after “concerned citizens” reported suspicious activity. After police determined that Abdo was wanted on the child pornography charge and was AWOL, they searched his motel room.

The case is now being investigated by the FBI, the Army’s Criminal Investigation Division and the Killeen police department.

“We’re looking into all aspects of his background to determine his intentions,” Vasys said.

Staff researcher Julie Tate contributed to this report.