US forces have captured the second suspect for his alleged role in the 2012 Benghazi attacks that killed four Americans, including Ambassador to Libya Christopher Stevens, the White House announced.

“On my orders, United States forces captured Mustafa al-Imam in Libya,” President Donald Trump said in a statement on Monday.

“To the families of these fallen heroes: I want you to know that your loved ones are not forgotten, and they will never be forgotten.”

The US government has a video of al-Imam present at one of the two sites of the attacks that killed four Americans, an administration official told CNN.

He said the US had been monitoring the terrorist operative’s location for some time.

Once al-Imam was captured, he was flown to a US Navy ship. He will be transferred to Washington for federal prosecution, the official added.

“The United States will continue to support our Libyan partners to ensure that IS and other terrorist groups do not use Libya as a safe haven for attacks against United States citizens or interests, Libyans, and others,” Trump said in his statement.

Attorney General Jeff Sessions on Monday vowed that al-Imam “will face justice in federal court for his role in the attack”, CNN reported.

“We will never forget those we lost — Tyrone Woods, Sean Smith, Glen Doherty, and Ambassador Christopher Stevens — four brave Americans who gave their lives in service to our nation. We owe it to them and their families to bring their murderers to justice,” Sessions said.

al-Imam’s capture comes as the attack’s suspected mastermind, Ahmed Abu Khatallah, is currently on trial in Washington.

Khatallah faces 18 charges related to the deadly violence that began on September 11, 2012.

During the attack, assailants armed with automatic weapons and rocket-propelled grenades first blasted through the main diplomatic mission before setting it ablaze.

Stevens and State Department information officer Smith died there. A coordinated mortar assault on a nearby annex killed Woods and Doherty, both CIA contractors and former US Navy SEALs.

Ambassador Stevens choked to death by thick black smoke.