The Libyan militant charged in the 2012 Benghazi attacks that killed four Americans pleaded not guilty to conspiracy in his initial appearance in U.S. federal court Saturday.

A grand jury indictment says Ahmed Abu Khattala took part in a conspiracy to provide material support and resources to terrorists in the attacks that killed U.S. Ambassador Chris Stevens and three other Americans.

U.S. special forces captured Abu Khattala in Libya two weeks ago, marking the first breakthrough in the investigation. Officials had been questioning Abu Khattala aboard a Navy ship that transported him to the United States.

The prosecution reflects the Obama administration's stated position of trying suspected terrorists in the American criminal justice system even as Republicans call for Abu Khattala and others to be held at the U.S. detention facility in Guantanamo Bay, Cuba.

Critics say suspected terrorists don't deserve the legal protections afforded by the American court. The administration considers the civilian justice system fairer and more efficient.

US Benghazi Militant

Ahmed Abu Khattala, an alleged leader of the deadly 2012 attacks on Americans in Benghazi, Libya, was captured by U.S. special forces on, June 15. (The Associated Press)

Ahmed Abu Khattala, an alleged leader of the deadly 2012 attacks on Americans in Benghazi, Libya, was captured by U.S. special forces on, June 15. (The Associated Press)

Abu Khattala was flown early Saturday by military helicopter from the ship to a National Park Service landing pad in the Washington's Anacostia neighborhood, according to a U.S. official who was not authorized to discuss the transfer publicly and spoke on condition of anonymity.

A criminal complaint filed last year and unsealed after Abu Khattala's capture charges him with terror-related crimes. They include killing a person during an attack on a federal facility; that crime can be punishable by death.

Prominent figure among extremists

The violence in Libya on the 11th anniversary of the Sept. 11 attacks on the World Trade Center and the Pentagon quickly became a political controversy at home.

Republicans accused the White House, as the 2012 presidential election neared, of intentionally misleading the public about what prompted the attacks. The White House said Republicans were politicizing a national tragedy.

Abu Khattala was a prominent figure in Benghazi's circles of extremists. He was popular among young radicals and lived openly in the eastern Libyan city, spotted at cafes and other public places, even after the Obama administration publicly named him as a suspect.

US Benghazi Militant

U.S. Marshalls guard the area outside of the federal U.S. District Court in Washington Saturday for the court appearance of Ahmed Abu Khattala later in the day. Khatallah. (Jose Luis Magana/ Associated Press)

U.S. Marshalls guard the area outside of the federal U.S. District Court in Washington Saturday for the court appearance of Ahmed Abu Khattala later in the day. Khatallah. (Jose Luis Magana/ Associated Press)

He is accused of being a member of the Ansar al-Shariah group, the powerful Islamic militia that the U.S. believes was behind the attack.

He acknowledged in an interview with The Associated Press in January that he was present during the storming of the U.S. mission in Benghazi. But he denied involvement in the attack, saying he was trying to organize a rescue of trapped people.

In the attack, gunmen fired rocket-propelled grenades and stormed the mission, with many waving the black banners of Ansar al-Shariah.

Ambassador suffocates to death

The compound's main building was set ablaze. Ambassador Chris Stevens suffocated to death inside and another American was shot dead.

At the time, several witnesses said they saw Abu Khattala directing fighters at the site.

Later in the evening, gunmen attacked and shelled a safe house, killing two more Americans. No evidence has emerged that Abu Khattala was involved in the later attack.

Abu Khattala is one of just a few cases in which the administration has captured a suspected terrorist overseas and interrogated him for intelligence purposes before bringing him to federal court to face charges.

Those cases include Osama bin Laden's son-in-law, Sulaiman Abu Ghaith, who was arrested in Jordan in March 2013 and turned over to U.S. agents. A jury in New York City convicted him in March of conspiring to kill Americans.