A cemetery worker walks next to fresh graves on September 25, 2013 after the funeral for Selima Merali, 41, and daughter Nuriana Merali, 15, who were killed in the attack by gunmen at the Westgate mall in Nairobi, Kenya. The country is observing three days of national mourning as security forces begin the task of clearing and securing the Westgate shopping mall following a four-day siege by militants.
Credit: Uriel Sinai

For the first time, the leader of the Somali militant group Al Shabaab has claimed responsibility for the mall shooting in Nairobi, Kenya, that killed 72 people this week.

Ahmed Godane, also known as Mukhtar Abu al-Zubayr, posed comments on the Shabaab-linked website Somalimemo.net on Wednesday, warning of further violence, Reuters reported.

He said the attack was retaliation for Kenya’s military action two years ago in Somalia against the insurgents, according to Reuters.

“Take your troops out or prepare for a long-lasting war, blood, destruction and evacuation,” Godane said, the news service reported.

Also on Wednesday, the Associated Press reported that Al Shabaab sent an email claiming foreigners inside the mall were a "legitimate target" during the four-day siege that began Saturday at noon.

The group's email exchange with AP suggested the militants rounded up hostages, freeing the Muslims and killing the "Kuffar," or non-believers.

"The Mujahideen carried out a meticulous vetting process at the mall and have taken every possible precaution to separate the Muslims from the Kuffar before carrying out their attack," an email said, AP reported.

It's believed at least 18 foreigners died inside the mall as armed insurgents held Kenyan security forces at bay for more than 80 hours.

More from GlobalPost: Kenya's 'island of peace' shattered by Westgate attack

The dead were from Canada, France, Australia, the Netherlands, Peru, India, Ghana, South Africa, the United Kingdom, Trinidad and China.

Al Shabaab said the Kenyan government should accept responsibility for the death toll.

Kenyan troops had helped African Union forces squeeze Al Shabaab from strongholds inside Somalia, culminating in October 2011.

It was believed the terrorist group had all but dissolved until they laid waste to the Westgate Shopping Mall.

Now, the search for more bodies, clues and reasons begin.

American, British and Israeli investigators have arrived in Kenya to help local authorities sift through the rubble, The Guardian reported.

One Kenyan official described the attackers as "multi-national." Kenya's foreign minister, Amina Mohamed, told PBS NewsHour that "two or three" American men and one British woman are believed to have been involved, though that information has yet to be confirmed.

"The investigators will be looking to see what information they can extract to identify the terrorists and their nationalities, including DNA tests," said a senior official from the National Disaster Operation Centre, according to The Guardian.

More from GlobalPost: Is Al Shabaab stronger than ever after Kenya mall attack?

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