A group that monitors extremism says Islamic State group supporters have published a letter said to be from three women who attacked a Kenya police station over the weekend. The letter pledges allegiance to ISIS.

All three women died in Sunday's attack in Mombasa, Kenya's second largest city. No group has claimed responsibility.

SITE Intelligence said late Monday that a Twitter user described the letter as the "final message of the brave sisters who struck fear in the heart of the kaafirs (disbelievers) at Mombasa police station on 9/11."

The head of the national Anti-terrorism Police Unit, Munga Nyale, says they cannot immediately establish the letter's authenticity.

Kenya over the past year has raised concerns that the Islamic State group is trying to gain a foothold in East Africa.