The bodies of four Americans killed in the US consulate attack in Libya have been flown back home.
President Barack Obama and Vice President Joe Biden earlier visited families of the victims.
"Their sacrifice will never be forgotten, we will bring to justice to those who took them from us."
- US President, Barack Obama
Obama and Biden joined Secretary of State Hillary Clinton and Secretary of Defence Leon Panetta to express condolences in a private waiting area at the Andrews Air Force Base terminal as the bodies arrived.
Chris Stevens, the US ambassador in Libya, and three others were killed on Tuesday after the US consulate in Benghazi came under attack during violent protests over an anti-Islam video.
Also killed were Americans Sean Smith, Glen A. Doherty and Tyrone S. Woods. Doherty and Woods were former Navy SEALS.
Libyan authorities have already made four arrests in the investigation into the attack, the deputy interior minister said.
"Four men are in custody and we are interrogating them because they are suspected of helping instigate the events at the US consulate," Wanis Sharif told the Reuters news agency on Thursday.
He gave no further details.
President Obama, speaking at the ceremony in Andrews Air Force base, said: "They had a duty and they believed in it, they lived the American ideal with courage, the hope and the ideals of the fundamental American beliefs. That is who they were and who we are.
"Their sacrifice will never be forgotten, we will bring justice to those who took them from us.
"We will continue to protect all Americans serving overseas, making it clear justice will come for those who harm America." he said.
Clinton said in her address: "The people in Libya, Egypt and Yemen did not replace the tyranny of dictators for the tyranny of the mob.
"We owe it to the four men who died that we carry on leading the world and strengthen the hand of democracy."
Al Jazeera's Rosalind Jordan, reporting from Washington, said: "The secretary of state to took the time to tell people that it is not the time to engage in violence, while thepresident again stressed those responsible will be brought to justice."
Stevens and the others died after gunmen attacked the US consulate and a safe house refuge in the eastern city of Benghazi.
Obama has pledged to bring to justice those responsible for the attack, which US officials said may have been planned in advance. Clinton said Washington had nothing to do with the video, which she called "disgusting and reprehensible".