The Obama administration, roiled by the first killing of a US ambassador in more than 30 years, is investigating whether the assault on the US consulate in Libya was a planned terrorist strike to mark the anniversary of the September 11, 2001, attacks and not a spontaneous mob enraged over an anti-Islam YouTube video. President Barack Obama vowed in a White House address that the US would "work with the Libyan government to bring to justice" those who killed Ambassador Chris Stevens and three other Americans. Related stories: Obama: Make no mistake, justice will be done US ambassador, staff killed in embassy attack Egyptians angry at film scale US embassy walls Intelligence officials said the attack on the Benghazi consulate was "too coordinated or professional to be spontaneous," according to a US counterterrorism official. The official spoke on condition of anonymity because he was not authorized to discuss the incident publicly. National Security Council spokesman Tommy Vietor said it would be premature to "ascribe any motive to this reprehensible act." Earlier Wednesday Official sources in Washington told CNN that the attack on the US embassy in Benghazi in which Ambassador Christopher Stevens and 3 embassy employees were killed was planned, and that the attackers used the protest outside the consulate as a diversion. Sources also noted that the attack was the work of roughly 20 militants, prepared for a military assault "it is rare that an RPG7 is present at a peaceful protest." Revenge for assassination? "According to our sources, the attack against the consulate had two waves. The first attack led to US officials being evacuated from the consulate by Libyan security forces, only for the second wave to be launched against US officials after they were kept in a secure location." Meanwhile, sources within the Quilliam Foundation in London believe a pro-al Qaeda group was responsible for the attack. They believe the attack was carried out as revenge over the death in June of the terror group Abu Yahya al-Libi. "An attack like this would likely have required preparation. This would not seem to be merely a protest which escalated," the source said. Make no mistake. Justice will be done," a somber Obama pledged at the White House, Secretary of State Hillary Rodham Clinton at his side. He ordered increased security at US diplomatic missions overseas, particularly in Libya, and said he condemned "in the strongest possible terms the outrageous and shocking" attack. Clinton said she was particularly appalled that the attack took place in Benghazi, which the US had helped liberate from dictator Moammar Gaddafi during the Arab Spring revolution in Libya this year. The aftermath of the two attacks also stirred the US presidential campaign, where until Wednesday, foreign policy had taken a back seat to the struggling economy. Obama spoke shortly after the Republican presidential nominee, Mitt Romney, criticized the administration for statements issued before and after the Cairo attacks that expressed sympathy for those insulted by the video. "I also believe the administration was wrong to stand by a statement sympathizing with those who had breached our embassy in Egypt instead of condemning their actions," Romney told a morning news conference. "It's never too early for the United States government to condemn attacks on Americans and to defend our values." Stevens, a 52-year-old career diplomat, was killed when he and a group of US employees went to the consulate to try to evacuate staff as the building came under attack by a mob wielding guns and rocket propelled grenades. Stevens is the first US ambassador to be killed in an attack since 1979, when Ambassador Adolph Dubs was killed in Afghanistan. Follow Ynetnews on Facebook and Twitter Receive Ynetnews updates directly to your desktop