President Barack Obama says the United States will be resolute in protecting its embassies while attending a ceremony marking the return of the bodies of four Americans killed in Libya this week.

President Obama said these are “difficult days” for the United States but he said America will never stop working for the world and for the freedom of others.

Secretary of State Hillary Clinton and Vice President Joe Biden also attended Friday's ceremony, where the bodies of U.S. Ambassador to Libya Christopher Stevens and three of his staffers arrived at a U.S. military air base near Washington.

The four Americans were killed at the U.S. consulate in Benghazi on Tuesday after protesters upset over a film mocking the Prophet Muhammad attacked the building.

Clinton paid tribute to the four men and called the violence against American diplomatic facilities “totally unacceptable.” She said the people of Egypt, Libya, Yemen, and Tunisia “did not trade the tyranny of a dictator for the tyranny of a mob.”

She also called the film mocking the Prophet Muhammad “awful” and said the United States had nothing to do with it.

U.S. authorities have been investigating whether the attack was carried out by militants who took advantage of the anti-U.S. protest. Attorney General Eric Holder has cut short a trip abroad to return to the United States and oversee the investigation.

On Friday, White House spokesman Jay Carney said the administration had no “actionable intelligence” in advance about the attack on the Benghazi facility.