WASHINGTON/BENGHAZI, Libya — President Barack Obama branded the killing of the U.S. ambassador to Libya and three other Americans an “outrageous attack” on Wednesday and vowed to track down the perpetrators, while ordering a tightening of diplomatic security worldwide.

The ambassador, Christopher Stevens, and the other Americans were killed after Islamist gunmen attacked the U.S. consulate and a safe house refuge in Benghazi on Tuesday night. The attackers were part of a mob blaming America for a film they said insulted the Prophet Mohammad.

The violence in the eastern city, a cradle of Libya's U.S.-backed uprising against Muammar Gaddafi last year, came on the anniversary of al Qaida's attacks on the United States on Sept. 11, 2001. Another assault was mounted on the U.S. embassy in Cairo in which protesters, who included Islamists and teenage soccer fans, tore down and burned a U.S. flag.

U.S. officials said the Benghazi attack may have been planned in advance and there were indications that members of a militant faction calling itself Ansar al Sharia — which translates as Supporters of Islamic Law — may have been involved.

They also said some reporting from the region suggested that members of Al-Qaida's north Africa-based affiliate, known as Al Qaida in the Islamic Maghreb, may have been involved.

“It bears the hallmarks of an organized attack,” one U.S. official said. However, some U.S. officials cautioned against assuming that the attacks were deliberately organized to coincide with the Sept. 11 anniversary.

The violence in Benghazi and Cairo threatened to spread to other Muslim countries on Wednesday. A U.S. official said Washington had ordered the evacuation of all U.S. personnel from Benghazi to Tripoli and was reducing staffing in the capital to emergency levels.

Police fired tear gas at angry demonstrators outside the U.S. embassy in Tunisia and several hundred people gathered in front of the U.S. embassy in Sudan. In Morocco, a few dozen protesters burned American flags and chanted slogans near the U.S. consulate in Casablanca.

Obama vowed Wednesday to bring the Benghazi killers to justice. He called the attack “outrageous and shocking” but insisted it would not threaten relations with Libya's new elected government.

Security stepped up

The President said he had ordered an increase in security at U.S. diplomatic posts around the globe. A U.S. official said a Marine anti-terrorist security team was being sent to Libya to boost security there.

Mitt Romney criticized the President's response to the crisis. He said the timing of a statement from the U.S. embassy in Cairo denouncing “efforts by misguided individuals to hurt the religious feelings of Muslims” made Obama look weak as protesters were attacking U.S. missions. He said it was “disgraceful” to be seen to be apologizing for American values of free speech.

Obama's campaign accused Romney of scoring political points at a time of national tragedy.

Images of ambassador Stevens purportedly taken after he died circulated on the Internet. One image showed him being carried, with a white shirt pulled up and a cut on his forehead.

Libya's Deputy Interior Minister Wanis Al-Sharif said U.S. staff were rushed to a Benghazi safe house after the initial attack on the consulate and an evacuation plane with U.S. commando units then arrived from Tripoli to evacuate them from the safe house.

“It was supposed to be a secret place and we were surprised the armed groups knew about it. There was shooting,” Sharif said. Two U.S. personnel were killed there, he said. Two other people were killed at the main consular building and between 12 and 17 wounded.

Witnesses said the mob included tribesmen, militia and other gunmen. Hamam, a 17-year-old who took part in the attack, said Ansar al-Sharia cars arrived at the start of the protest but left once fighting started.

“The protesters were running around the compound just looking for Americans, they just wanted to find an American so they could catch one,” he said.

“We started shooting”

“We started shooting at them, and then some other people also threw hand-made bombs over the fences and started the fires in the buildings,” he said.

“There was some Libyan security for the embassy outside but when the hand-made bombs went off they ran off and left.” Hamam said he saw an American die in front of him in the mayhem that ensued. He said the body was covered in ash.

The “Innocence of Muslims,” the film blamed for provoking the violence, was the work of a California-based Israeli filmmaker Sam Bacile. It portrays Mohammad as a fool, a philanderer and a religious fake and in one clip posted on YouTube, he was shown in an apparent sexual act with a woman.

“Small and savage group”

Secretary of State Hillary Clinton said the attack was the work of a “small and savage group.”

Abdel-Monem Al-Hurr, spokesman for Libya's Supreme Security Committee, said Libyan security forces came under heavy fire and “were not prepared for the intensity of the attack.”

Tributes poured in to honor Stevens, who said in a video posted on the embassy website of his involvement in the Libyan revolution: “I was thrilled to watch the Libyan people stand up and demand their rights.”

Stevens, who was 52, grew up in California, graduated from Berkeley and worked in North Africa as a Peace Corps volunteer.

He taught English in Morocco before joining the foreign service, where he worked in the Middle East and North Africa.