A suicide bomber killed a Turkish guard in what U.S. officials called a terrorist attack on the U.S. Embassy in Ankara on Friday, but current and former diplomats credited increased security with preventing more deaths.

“A suicide bombing on the perimeter of an embassy is by definition an act of terror,” White House press secretary Jay Carney said. “However, we do not know at this point who is responsible or the motivations behind the attack.”

Turkey, a key NATO ally, blamed the assault on a left-wing militant group with roots in the Marxist movements of the 1970s and a history of attacks on Turkish and American security targets.

There was no immediate indication of a link either to al-Qaeda or to militants in neighboring Syria. But analysts said the group identified by Turkish officials,
the outlawed Revolutionary People’s Liberation Party-Front, or DHKP/C, has received protection and help from within Syria.

A U.S. official briefed on the bombing said there is no reason to doubt that the attack was launched by DHKP/C. But the official, who spoke on the condition of anonymity because he was not authorized to talk to the media, said investigators are considering whether the bombing was motivated by the Syrian civil war.

The United States and Germany have deployed Patriot missiles along Turkey’s border with Syria at Ankara’s request, drawing rebukes from Syria and Iran.

Friday’s attack, shortly after 1 p.m. in the Turkish capital, was the second fatal assault on a U.S. diplomatic mission in the last five months, and came on Hillary Rodham Clinton’s last day as secretary of state. “We live in very complex and even dangerous times, as we saw again just today at our embassy in Ankara,” Clinton told State Department employees in her final address Friday.

Unlike the mob assault in Benghazi, Libya, the lone Turkish attacker was unable to get farther than a checkpoint at the far perimeter of the embassy compound, the State Department said.

Republicans charged that the White House was slow to brand the Sept. 11 Libya assault as terrorism and may have tried to cover up potential links to al-Qaeda that could tarnish President Obama’s national security credentials. An independent report faulted Libyan security guards for letting the attackers overrun the Benghazi compound and said the temporary post was too lightly defended.

By contrast, U.S. officials praised the actions of Turkish contract guards and said security precautions installed over the last decade kept the bomber far from embassy staff members.

The bomber wore a suicide vest and blew himself up at a security checkpoint some distance from the embassy building, U.S. officials said. The explosion killed the Turkish guard closest to the bomber, but two other guards were protected by bulletproof glass, State Department spokeswoman Victoria Nuland said.

“The level of security protection at our facility in Ankara ensured that there were not significantly more deaths and injuries,” Nuland said.

Nuland said the United States will cooperate with Turkey in the investigation of the blast.

A Turkish woman identified as television journalist Didem Tuncay, 38, was injured, apparently by flying debris.

The State Department warned Americans in Turkey to avoid the embassy, as well as U.S. consulates in Istanbul and Adana, for now.

“From today’s event, it is clear that we both suffer from this terrible, terrible problem of today’s world. We are determined after events like this even more to cooperate together until we defeat this problem together,” U.S. Ambassador Francis J. Ricciardone Jr. said.

Turkish Prime Minister Recep Tayyip Erdogan said the attack was intended to undermine Turkish “peace and prosperity.”

“We will stand firm and we will overcome this together,” he said.

James Jeffrey, a former U.S. ambassador to Turkey, said the DHKP/C has tried to attack U.S. diplomatic facilities before, but the group is not seen as a major threat to Western interests.

Ozgur Unluhisarcikli, who heads the Ankara office of the German Marshall Fund of the United States, said Syria began providing haven to the DHKP/C in the 1980s. He said he suspects a Syrian hand in Friday’s attack because of the deterioration of Syrian-Turkish relations since the start of the Syrian civil war.

“What we saw happening today is a continuation of the proxy war between Turkey and Syria,” Unluhisarcikli said.