Three North Korean doctors were killed by unidentified men inside their apartment in the northeastern Nigerian state of Yobe, officials said.

Men armed with knives scaled the fence of an apartment housing the three doctors, who were working at a government-run hospital, and slit their throats while the third was beheaded, Yobe State police commissioner Sanusi Rufa’i told AFP.

The victims, initially identified as Chinese and South Koreans, were found on Sunday morning, after people became worried that they were not answering the door.

An official at the General Hospital in Potiskum told the Associated Press said that the doctors’ apartments had no security guards, and that they had routinely travelled through the town in taxis without police escort.

The attacks occurred in the town of Potiskum, a stronghold of the Islamist group Boko Haram, but no group has claimed responsibility. Authorities have yet begun an investigation to identify the assailants and whether they came with guns.

“It is still premature to point any accusing fingers but we have commenced an investigation to unravel the killings,” said Rufa’i.

Sunday’s attack was the latest in a spate of killing of foreigners, especially Chinese, in the country’s volatile region, AFP reports.

On Friday, nine polio vaccination workers – all said to have been women – were shot dead in two Nigerian polio clinics.

More than 600 people were killed in 2012 by the group, whose name means “Western education is sacrilege”, in attacks aimed at overthrowing the government and creating an Islamic state in Nigeria’s predominantly Muslim north.