Two explosions rocked an ammunition depot belonging to Pakistan's Counter-Terrorism Bureau (CTB) on Monday, killing at least 13 and wounding more than 50, police said.
Reuters quoted the regional police chief as saying the blast-hit counter-terrorism bureau was located in northwest Swat Valley, which had long been controlled by "Islamist militants" before they were driven out in a military operation in 2009.
Later, a provincial police spokesman said: "The ammunition depot caught fire probably due to an electrical short, there is no evidence of an attack from outside so far."
The spokesman added that some other aspects of the blasts were being investigated.
Hayat said most of the dead were counter-terrorism police officers, as well as a woman and her child who happened to pass near the building at the time of the explosion.
Critical condition
Pakistani medical sources said the wounded were taken to hospitals for treatment, noting that some of them were in critical condition.
AFP described the incident as an attack and quoted a counter-terrorism police officer as saying the blasts caused the targeted building to collapse completely.
The agency said no one had claimed responsibility for the attack so far, but added that two attacks on two major police stations since the start of the year had been attributed to the Pakistani Taliban.