“This is one of the compounds where we have been making steady security upgrades over the last decade, and, in fact, the attack was at one of the exterior compound access sites, so it was far from the main building,” Nuland said. “And it was a result of the way that it was hardened that we only lost the one local security guard.”

From the highest levels, the Obama administration was quick to call the Ankara blast a terrorist attack. One of the criticisms of the administration’s handling of Benghazi was its vacillating between calling it a terrorist attack and a spontaneous offshoot of protests, such as ones that simultaneously targeted U.S. posts in Indonesia, Egypt and Tunisia. It was finally determined to have been an Islamist militant attack.

Vice President Joe Biden, meeting in Berlin with German Chancellor Angela Merkel, welcomed her sympathy for Friday’s attack, adding that although he didn’t have the details, “it’s characterized obviously as a terrorist attack on our embassy in Ankara.”

At the White House, spokesman Jay Carney said that a suicide bombing is by definition an act of terror, but that “we do not know at this point who is responsible or the motivations behind the attack.”

The Turkish interior minister said publicly that the attacker was a member of a radical leftist group. Some terrorism experts called his statements premature, saying that such tactics were more typically used by al Qaida-style extremists. U.S. authorities seemed reluctant to endorse the Turks’ official version until the completion of a full investigation.

“The Turkish side believes that the attacker was a member of an outlawed leftist organization. I think we need to let the Turkish side investigate,” Nuland said.

The bombing disrupted Secretary of State Hillary Clinton’s final day in office. She started what was to have been a day of farewell events with a briefing on the Ankara blast, followed by phone conversations with the U.S. ambassador to Turkey, Francis Ricciardone, and Turkish Foreign Minister Ahmet Davutoglu.

Incoming Secretary John Kerry also was briefed via his staff, Nuland said. Kerry was sworn in Friday afternoon.

Lesley Clark contributed to this article.