WASHINGTON — An Iraqi man who worked as a translator for the American military before moving to the United States in 2009 was arrested on Thursday in Texas by F.B.I. agents who say he pledged allegiance to the leader of the Islamic State last year and misled them about his travels to Syria.
The Justice Department said that the man, Bilal Abood, 37, a naturalized American citizen who lives in Mesquite, a Dallas suburb, was charged with lying to an agent after he denied last month that he had pledged loyalty to Abu Bakr al-Baghdadi, the head of the self-described Islamic State.
An examination of his computer, seized in July under a search warrant, showed that on June 19 he had written on Twitter, “I pledge obedience to the Caliphate Abu Bakr al-Baghdadi,” officials said. The caliphate is the unified Muslim state that Mr. Baghdadi claims to have created, which is also known as ISIS or ISIL.
Officials said the timing of the arrest of Mr. Abood, who had been under F.B.I. scrutiny for more than two years, reflected increasing wariness about the possible threat posed by known devotees of the Islamic State inside the United States. The group has called on its followers to carry out attacks in its name, and two Islamic State supporters were shot dead by the police on May 3 in Garland, Tex., after they opened fire on a contest for Prophet Muhammad caricatures.
In March 2013, Mr. Abood was prevented from boarding a flight at Dallas-Fort Worth International Airport, and he initially told agents that he had planned to visit his family in Iraq. A few days later, he acknowledged that he had actually intended to travel to Syria to fight against the government of Bashar al-Assad.
The following month, the charging documents say, the F.B.I. learned from an informant that Mr. Abood was watching videos about the Islamic State and had said that he wanted to support it.
He then traveled to Mexico and went from there through several countries before reaching Turkey, the usual transit country to Syria, the documents say.
When he returned to Texas in September 2013, he told F.B.I. agents that he had stayed in a camp of the Free Syrian Army, an opposition coalition supported by the United States, but “became frustrated with a lack of action” and decided to come home.
About 10 months passed before investigators searched his computer and found his pledge to Mr. Baghdadi.
After that discovery, another nine months passed before agents asked Mr. Abood about the pledge, leading to what the authorities called his false statement.
Last summer, Mr. Abood told the ABC affiliate in Dallas that agents were harassing him because he had refused to cooperate with them.
He faces a possible maximum sentence of eight years in prison if convicted on the false statement charge.