A fighter of the Islamic State group waves the terrorist group’s flag from inside a captured government fighter jet following the battle for the Tabqa air base, in Raqqa, Syria. (Uncredited/AP)

Federal authorities announced Saturday that they had arrested a Northern Virginia man who they say planned to join the Islamic State in Syria, intercepting him just before he departed from Richmond International Airport for the first leg of his journey.

They also charged a man who gave him a ride, alleging he was in on the plan and lied to FBI agents about it.

In court papers, authorities said Joseph Hassan Farrokh, 28, and Mahmoud Amin Mohamed Elhassan, 25, both of Woodbridge, had been unknowingly working with law enforcement informants for months, plotting to have Farrokh join the Islamic State in Syria while the FBI monitored their communications.

Investigators took Farrokh into custody Friday as he walked to his departure gate, charging him with attempting to provide material support to terrorists, according to court documents. They arrested Elhassan, who had given Farrokh a ride to Richmond, in Woodbridge some time later, charging him with aiding and abetting Farrokh’s plot, the documents say.

Relatives of Farrokh could not immediately be reached. A man who identified himself as Elhassan’s brother said Elhassan had never shown signs of supporting the Islamic State, and he did not believe the allegations.

“I 100 percent believe my brother is an innocent person, and by the end of the story, they’re going to find what a big mistake they did to him,” Elhassan’s brother said. Asked to spell his name, the brother said he was driving and had to discontinue the phone call.

If the allegations are true, the case would mark the latest example of an American buying into the Islamic State’s agenda and trying to lend a hand. Federal prosecutors have charged more than 70 men and women across the country with crimes related to the Islamic State, and December seemed to mark an especially busy month on that front.

The man who identified himself as Elhassan's brother said Elhassan was a student at Northern Virginia Community College and had a license to drive a cab. The man, 45, said Elhassan came from a family of “simple, normal, middle class, Middle Eastern Muslims” who did not support the Islamic State.

“Personally, we believe that ISIS is something bad, something wrong, and we don’t support them,” the man said, using an acronym for the Islamic State. “We don’t deserve this. We work so hard for this country.”

The man said he had not yet been able to talk to his brother.

Less could be learned immediately about Farrokh. An affidavit signed by U.S. Secret Service Agent Walter T. Johnson Jr. says Farrokh was born in Pennsylvania and had been living in Woodbridge since July. It says Elhassan was originally from Sudan but became a legal, permanent U.S. resident in 2012 and also lived in Woodbridge.

It remains unclear precisely how or when agents were tipped to Farrokh’s and Elhassan’s alleged intentions, but the affidavit says Elhassan introduced Farrokh to an informant in August, and agents were able to keep tabs on the men via that informant and others in the months that followed.

The affidavit alleges that Farrokh said at various points that he had been wanting to travel to Syria for a year, and that he wanted to die a martyr for Allah.

Based on the affidavit's account, Farrokh seemed wary of law enforcement thwarting his plans. The FBI recorded a meeting between Farrokh and two confidential informants — one who Farrokh thought was an Islamic State facilitator — in November, and Farrokh talked of not wanting to go to jail for 15 years, according to the affidavit. The affidavit claims the informants told Farrokh he did not have to talk if he was not comfortable, and Farrokh suggested they swear an oath that those working with non-Muslims would suffer the curse of Allah.

Later, Farrokh questioned why it was necessary that he formally declare an oath of allegiance in the United States, where he might be prosecuted for it, and why he had to give them copies of his passport and other documentation, which would prove his intent to travel. He also allegedly mentioned to Elhassan the arrest of 25-year-old Emanuel Lutchman, who authorities said was plotting a New Year’s Eve attack for the Islamic State. According to an affidavit in that case, Lutchman was conspiring with people working for the FBI, one of whom gave him $40 to buy ski masks, zip ties, knives and other supplies.

The informants, according to the affidavit, gave Farrokh the chance to back out at various points, but he ultimately pressed forward.

Farrokh's family seems not to have known about his plan. He told an informant that he had told them he was planning to go to Saudi Arabia to study, and he feared that if he lingered too long in Jordan — where he planned to fly before heading to Syria — his family might realize what he was up to. The affidavit indicates Farrokh was married but offers no details about his wife.

In one phone call, according to the affidavit, Farrokh’s mother told him he sounded aggressive, and he responded that he had asked Allah to destroy Christians because they are the enemies of humanity.

Farrokh ultimately purchased a ticket to fly out of Richmond to Jordan, with a layover in Chicago, according to the affidavit. He told an informant he planned to trim and style his beard so he would look more American. Elhassan, according to the affidavit, knew of Farrokh’s plans, and he and Farrokh left for Richmond in Elhassan’s taxi just before 8 a.m. Friday.

Agents arrested Farrokh soon after he passed airport security, according to the affidavit. They later took Elhassan into custody in Woodbridge, and he told agents — falsely, according to the affidavit — that Farrokh was flying to a friend’s funeral in California, and he was leaving out of Dulles International Airport. Elhassan’s brother said investigators searched the family’s home.

Both Farrokh and Elhassan are scheduled to appear in federal court in Alexandria Tuesday.