Updated at 3:37 p.m. ET

An American citizen suspected of becoming a sniper and weapons trainer for the Islamic State has been brought back to the United States and charged with aiding the terrorist group.

The charges against Ruslan Maratovich Asainov are contained in a criminal complaint unsealed Friday in federal court in the Eastern District of New York.

The government says Asainov is a naturalized U.S. citizen born in Kazakhstan who was captured by American allies in Syria. It's not clear when Asainov was detained. He was recently transferred to FBI custody and arrived back in the U.S. on July 18 via New York's John F. Kennedy Airport.

Asainov was scheduled to make his initial court appearance on Friday. The government is arguing for his continued detention, pending trial.

Asainov is the latest American citizen to be captured by U.S. allies in Syria, transferred to U.S. custody and then repatriated to face charges over his suspected ties to the Islamic State.

In 2018, for example, a Michigan man, 28-year-old Ibraheem Musaibli, was detained in Syria, brought back to the U.S. and charged with providing material support to the terrorist group.

"The United States is committed to holding accountable those who have left this country in order to fight for ISIS," said a Justice Department spokesman, Marc Raimondi.

"When supported by the facts and the law, the Department of Justice will pursue criminal charges against such individuals. The United States believes that every country should take responsibility for its citizens who have tried or succeeded in joining ISIS."

But in at least one instance, the Trump administration has balked at bringing a citizen back to face trial.

It held an unidentified America-citizen as an enemy combatant for more than a year at a U.S. military facility in Iraq before finally freeing him. The American Civil Liberties Union had challenged his detention in federal court.

The sniper

Asainov capture and repatriation appeared to bring to an end what the government presents as a five-year odyssey of soldiering on the Islamic State's behalf.

It began in December 2013, court papers say, when Asainov bought a one-way ticket and boarded a flight at JFK airport and flew to Istanbul, Turkey. Once there, he crossed the border into Syria and joined the Islamic State, also known as ISIS.

He fought as a sniper for the group and by early 2015, prosecutors say, was "touting his allegiance to ISIS," and proclaiming "we are the worst terrorist organization in the world that has ever existed."

Over time, he rose through the group's ranks and was became responsible for training other Islamic State members how to use weapons, prosecutors say.

"The defendant trained as a sniper and eventually attained a high enough rank within ISIS's sniper corps that ISIS fighters and supporters referred to him as a sniper 'emir,'" the government's detention memo says. "The defendant further helped to establish training camps for ISIS fighters to train the fighters in the use of weapons."

The informant

A confidential informant working with investigators helped build the case against Asainov, according to court papers. The two communicated through a cell phone messaging application while Asainov is alleged to have been in Syria.

Asainov tried to recruit the informant to travel to Syria and join the Islamic State, according to court documents, telling him that he could work for the terrorist group's media operations and would be provided with housing, food and a $50 stipend per month.

In March 2015, Asainov asked the informant to send him $2,800 so that he could buy a scope for his rifle, according to court papers.

The informant did not provide the money.

A month later, Asainov told the informant that he'd been fighting for a year since he arrived in Syria and that "it's been pretty hot since they started bombing us." He also sent two photographs of himself dressed in combat fatigues and holding what court papers described as a "large-caliber assault rifle, fitted with a scope."

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