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Turkish and American officials were guarded about who was behind Friday鈥檚 attack outside the US embassy complex in Ankara that killed a Turkish security guard.

But the State Department 鈥� still sensitive to charges that it was slow to call last September鈥檚 attack on the US mission in Benghazi, Libya, a terrorist attack 鈥� lost no time in putting a label on the latest incident.

鈥淲e can confirm a terrorist blast at a check point on the perimeter of our embassy compound in Ankara, Turkey,鈥� State Department spokeswoman Victoria Nuland said in a statement issued shortly after the attack.

The blast at the embassy compound entrance, the work of a suicide bomber, injured several individuals in addition to killing the guard.

Although responsibility for the attack was still unclear Friday afternoon, speculation centered on a Turkish leftist group. Terrorism experts said possibilities range from Islamist extremists or Turkey鈥檚 Kurdish separatists to Iran and Syria.

Turkey鈥檚 Interior minister, Muammer G眉ler, told reporters that 鈥減reliminary information鈥� suggested the blast was the work of the Revolutionary People鈥檚 Liberation Party-Front, a leftist group that has carried out attacks on Turkish police stations in past years. Turkish police sources claimed that the bomber was a member of this group and had spent time in prison on terrorism-related charges.

This group has targeted American business interests in the past but has not been known to adopt other groups鈥� causes, says James Jeffrey, a former US ambassador to Turkey and Iraq who is now at the Washington Institute for Near East Policy. The group is listed by the United States as a terrorist organization.

But terrorism experts said other groups have carried out attacks on US interests in the past in Turkey and would have motivation for targeting the Ankara embassy.

Islamist extremists were behind the Benghazi attack and have carried out terrorist acts against Western sites in Turkey in recent years.

The US lists the Kurdish organization Kurdistan Workers鈥� Party, or PKK, as a terrorist organization and has aided Turkey in its battle with the group. But the PKK has not attacked US interests in its long history, experts note.

Then there are Iran and Syria. Iran, which US officials consider the world鈥檚 major state sponsor of terrorism, has been engaged for several years in a tit-for-tat covert war with the US over its advancing nuclear program. Syria is Iran鈥檚 principal client state in the Middle East, and Syria鈥檚 relations with both the US and Turkey are at a low point, with both countries supporting the ouster of Syrian President Bashar al-Assad.

President Obama ordered stepped-up security measures at American diplomatic missions shortly after the Benghazi attack, which claimed the lives of four Americans, including US Ambassador to Libya Christopher Stevens. The State Department, which was accused in congressional hearings of ignoring pleas for reinforced security in Libya before the Benghazi attack, is also reviewing security measures.

It was unclear Friday if security measures had recently been strengthened in Ankara and in Turkey in general, but Mr. Jeffrey says that the embassy compound in Ankara has long been 鈥渧ery well fortified.鈥�

Still, it would not come as a surprise to US officials that American installations in Turkey could be targeted, terrorism experts say.

In 2008, Al Qaeda sympathizers were blamed for an attack at the US consulate in Istanbul that killed three police officers and three attackers. In 2003, suicide bombings targeted the British consulate and two synagogues in Istanbul and left 58 people dead.

Some regional experts were quick to point out that tensions in the already-jittery region ratcheted up several notches this week after Israel bombed targets inside Syria for the first time since 2007. Israel said it was targeting arms shipments destined for Hezbollah 鈥� the pro-Iranian extremist group that experts say has developed assets inside Turkey.

Syria quickly warned it could carry out counterstrikes against Israel.

But other terrorism experts were cautious, saying it is unlikely that Friday鈥檚 attack could have been organized so soon after the Israeli airstrikes in Syria, which were carried out Wednesday.