U.S. Special Operations forces alongside the FBI apprehended one of the alleged suspects of the 2012 terrorists attacks in Benghazi, U.S. officials said.

The troops captured Ahmed Abu Khattala on Sunday during a secret raid in Libya, near Benghazi. The FBI and Special Operations had been planning the operation for months, The Washington Post reported.

Pentagon Press Secretary Rear Adm. John Kirby said Tuesday that Abu Khattala is now in U.S. custody "in a secure location outside Libya." He also added that all U.S. personnel involved left Libya safely and there were no civilian causalities during the raid.

Four U.S. citizens, including U.S. Ambassador Christopher Stevens, died during the attacks, in which militants launched grenades and other weapons at the U.S. Embassy compound, CNN reported.

Federal prosecutors filed sealed charges against Abu Khattalah last year for his involvement in the Benghazi attack. At least a dozen others suspected of taking part in the attacks have also been charged, according to the Post.

Abu Khattala -- the first of those suspected to be captured -- is expected to be in Washington for his arraignment.

Officials told The Post, on condition of anonymity, that Abu Khattala was "en route" to the U.S. but could not say where he was being held or when he will arrive.

Abu Khattala was branded as a terrorist in January by the State Department, which called identified him as a "senior leader" of the Benghazi sect of Ansar al-Sharia, a militant organization that arose after Moammar Gaddafi's regime fell in Libya in 2011.

A Senate Intelligence Committee report on Benghazi last January concluded that the attack was "likely preventable" based on the security shortfalls at the facility and prior warnings, according to CNN.

The Benghazi attacks occurred 11 years to the day after the Sept. 11 terrorists attacks on the World Trade Center and Pentagon in 2001, which resulted in the death of 2,977 people.

The attacks were first believed to be retaliation from an angry mob toward a video made in the U.S. that mocked the religion of Islam and the Prophet Mohammed. Officials would later determine that the incident was a terrorist attack.