At the nexus of Europe and Asia, and with deep roots in the Muslim and Christian spheres, Turkey has long been a boiling pot -- and, occasionally, a target.

In recent years, it has been site of many acts of political violence from groups such as leftist anarchists, Kurdish separatists, Islamists and al Qaeda.

Hasan Selim Ozertem, a security expert at the International Strategic Research Organization in Ankara, said Friday's attack could be related to recent arrests of DHKP-C members.

Since the beginning of January, 85 members of the group have been taken into custody, he said, adding that Turkish police have been closely focusing on the group over the past five years. The DHKP-C was established in the 1970s.

Ozertem said that one plausible theory is the group is trying to send a message to Turkish authorities by attacking the U.S. Embassy because it is near the Turkish parliament building.

DHKP-C has a track record as a "subcontractor" group for other militant outfits, and it is also believed to have relationships with states in the region such as Syria and Iran, Ozertem said.

The group has a relationship with the PKK, the Kurdistan Workers Party, which has been at odds with Turkey's government for some time. Ozertem said the attack could be linked to negotiations between the PKK and Turkish government.

Another possibility is that Syria or Iran could be involved, considering the recent deployment of Patriot missiles in Turkey as a defense against possible missiles from Syria.

The explosion occurred as about 400 U.S. military personnel are moving Patriot missile defense equipment to a Turkish base as part of an effort to defend the country from possible attack from Syria. The first battery became operational last Saturday in the city of Adana, NATO said, and more equipment arrived Wednesday in the port city of Iskenderun.

Erdogan, however, ruled out that Friday's attack had anything to do with Syria, according to an interview on the private Habertuk channel later reported by Andolu.

Of course, this may have to do primarily with the United States itself -- independent of anything involving Turkey or its government.

While the U.S. Embassy in Ankara has not seen this kind of incident in decades, in 2008 three police officers died in a shootout with assailants outside the U.S. Consulate in Istanbul.

Three attackers died in the incident, which the U.S. ambassador at the time called "an obvious act of terrorism." One of the attackers in that incident was believed to have trained with al Qaeda in Pakistan's Waziristan region.