NAIROBI, Kenya – Militants from the Somali-based Shabab group opened fire in Nairobi’s upscale Westgate mall complex on Saturday, committing mass slaughter and taking numerous hostages.

More than sixty civilians are dead, and the Kenya Red Cross has said over fifty are still missing.

The New York Times reported on Tuesday that Kenya’s President Uhuru Kenyatta announced government forces had succeeded in having “ashamed and defeated [the] attackers” after four days.

Concerns have been raised that some of the militants may have escaped the mall in the chaos following the initial attack, but Kenyatta declared that all those remaining in the mall were killed.

During the standoff, both the Shabab and the Kenyan government sent out Tweets suggesting their side was gaining ground over the other.

The attack was planned carefully. Officials report that a mall employee aided between ten and fifteen radicals in hiding military-grade machine guns in a shop in the days before the siege. Insurgents studied blueprints of the mall and rehearsed the attack. Further, the Shabab carefully selected a “team of English-speaking foreign fighters” to carry out the plan. Mr. Kenyatta claimed that “intelligence reports had suggested that a British woman and two or three American citizens may have been involved.” He acknowledged, however, that these reports could not be confirmed.

Al-Shabab is a Somali based Islamist organization that has formed a connection with al-Qaeda. The BBC recently described the group as follows:

“Al-Shabab means The Youth in Arabic. It emerged as the radical youth wing of Somalia’s now-defunct Union of Islamic Courts in 2006, as it fought Ethiopian forces who had entered Somalia to back the weak interim government.”

The group controls many of Somalia’s more rural areas.

Some Muslims were killed in the mall, but survivors have said that militants actively singled out non-Muslims for execution.

Recently, a Shabab member who spent his early life in Alabama, Omar Hammami, was killed by a Shabab faction. Hammami had criticized the Shabab for being too brutal towards other Muslims. The New York Times suggests that his murder may have been an impetus for the distinction between Muslims and non-believers.

Kenya began a three-day period of national mourning on Tuesday to mark the tragedy.