At least 13 policemen were killed by one of their own in Afghanistan's southern Helmand province.
A police source told the BBC that an infiltrator from the Taliban had allowed militants into the police station in the regional capital of Lashkar Gah last night.
The infiltrator fled the scene with the Taliban, he said.
A spokesperson for provincial governor Omar Zuak confirmed the incident, but did not provide details.
No group has officially claimed responsibility.
Lashkar Gah has come under increased pressure from the Taliban in recent months.
The city of 200,000 people is seen as strategically important, and the key to holding Helmand province.
The province is both a fertile farming region and a centre for opium production, bordering Pakistan on one end and covering major routes between cities.
Thousands of people fled the rural areas to shelter in Lashkar Gah last August, before the city itself came under attack in October.
The city was previously the scene of heavy fighting between the Taliban and Nato-led forces before their withdrawal in 2014.