Officials had earlier said that the bomber detonated a suicide vest at the checkpoint on the outer perimeter of the compound.
The guard who was killed was standing outside the checkpoint. The U.S. ambassador on Saturday attended his funeral in a town just outside of Ankara.
A Turkish TV journalist was seriously wounded and two other guards had lighter wounds.
DHKP-C's forerunner, Devrimci Sol, or Revolutionary Left, was formed in 1978 as a Marxist group openly opposed to the United States and NATO. It has attacked Turkish, U.S. and other foreign targets since then, including two U.S. military contractors and a U.S. Air Force officer.
The group, designated as a terrorist organization by the United States and other European allies, changed its name to DHKP-C in 1994.
Friday's attack came as NATO deployed six Patriot anti-missile systems to protect its ally Turkey from a possible spillover from the civil war raging across the border in Syria. The U.S., Netherlands and Germany are each providing two Patriot batteries.
Ozcan, the terrorism expert, said the Syrian regime, which had backed terrorist groups in Turkey, including autonomy-seeking Kurdish rebels, during the Cold War era and through the 1990s, had recently revived ties with these groups.
As Turkey began to support the Syrian opposition, Syrian President Bashar Assad's regime began to try "rebuilding its ties with these organizations," Ozcan said.
Radikal newspaper reported that the DHKP-C had recently been taking an interest in "regional issues," reviving its anti-American stance and taking on "a more pro-Assad position."
Former U.S. ambassador to Turkey Ross Wilson speculated that the masterminds of the embassy bombing may have been partly motivated by U.S.-Turkish policy on Syria.
"A successful attack would embarrass the Turkish government and security forces, and it would have struck at the United States, which is widely — if wrongly — thought to have manipulated the Erdogan government into breaking with Bashar al-Assad and supporting efforts to remove him from power," Wilson, director of the Dinu Patriciu Eurasia Center at the Washington-based Atlantic Council, wrote in an analysis. "That might rekindle public support for the group. Alas for DHPK/C, this seems unlikely."
Howard Eissenstat, a Turkey expert at St. Lawrence University in the United States, said the bombing showed that a "relatively isolated and obscure group" still has the capacity to cause havoc.
"They really fall outside of our comfortable narratives," Eissenstat wrote in an email to The Associated Press. "And they do seem to have been left in an ideological time warp. There is something distinctly cult-like about them."
The attack drew quick condemnation from Turkey, the U.S., Britain and other nations, and officials from both Turkey and the U.S. pledged to work together to fight terrorism.
It was the second deadly assault on a U.S. diplomatic post in five months.
On Sept. 11, 2012, terrorists attacked a U.S. mission in Benghazi, Libya, killing U.S. Ambassador Chris Stevens and three other Americans. The attackers in Libya were suspected to have ties to Islamist extremists, and one is in custody in Egypt.
U.S. diplomatic facilities in Turkey have been targeted previously by terrorists. In 2008, an attack blamed on al-Qaida-affiliated militants outside the U.S. Consulate in Istanbul left three assailants and three policemen dead.
Associated Press writers Ezgi Akin and Burhan Ozbilici and Christopher Torchia in Istanbul contributed to this report.
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