The gunmen who attacked an upscale mall in Kenya's capital, killing at least 67 people, likely died in the attack, an FBI official has said.

Al-Qaida-linked al-Shabab militants claimed responsibility for the Sept. 21 attack on Nairobi's Westgate Mall. Al-Shabab said the attack was in retaliation of Kenya sending its troops to Somalia to fight the militants.

Dennis Brady, the FBI Legal Attache in Nairobi, said in an interview posted Friday on the bureau's website: "We believe, as do the Kenyan authorities, that the four gunmen inside the mall were killed."

"Our ERT( Evidence Response Team) made significant finds, and there is no evidence that any of the attackers escaped from the area where they made their last stand," he said.

A very secure crime scene perimeter made an escape unlikely, he said.

"Additionally, had the attackers escaped, it would have been publicly celebrated and exploited for propaganda purposes by al-Shabab. That hasn't happened," he said in the posted interview.

A New York Police Department report on the attack raised the possibility the gunmen may have escaped.

The top State Department official for Africa, Linda Greenfield-Thomas, said in December the NYPD report does not reflect the U.S. government position.

FBI officials were on the scene from the first day, and their role is to facilitate, enable and assist the Kenyan investigation and prosecution, Brady said.

The crime scene following the attack was "very complicated," he said, because in the process of fighting the attackers, there were explosions and a fire that continued to burn, eventually causing a four-story structure to collapse into a pit that smoldered for weeks.

"It was a very difficult place to work. While ERT was doing its work, every now and then a propane tank would explode or vehicles on the edge of the collapse would fall in and catch fire," he said

Four individuals charged in court by Kenyan authorities in connection to the attack are directly connected to the individuals who physically carried out the attack, he said.

Investigations continue to identify the entire network that carried out the attack, he said.