At least 12 people have been killed in two explosions at a counter-terrorism office in north-west Pakistan, officials say.
Police said that there is no evidence of an attack and that it could have been as a result of an electrical fault igniting ammunition in the centre.
Most of the dead are police officers.
The explosion happened in the Swat valley, which was controlled by Islamist militants before they were forced out in 2009.
More than 40 people were injured and buildings have collapsed.
A spokesman for provincial police in Khyber Pakhtunkhwa, which neighbours Afghanistan, said that ammunition caught fire, "most probably due to an electric short-circuit".
"No evidence of an attack from outside has been established so far."
The Pakistani Taliban group has carried out several attacks targeting security forces in recent months, but has not claimed involvement in these explosions.
The regional chief of the counter-terrorism department Sohail Khalid told Reuters that the explosions did not appear to be an act of terrorism.
"There was a store where we had a huge quantity of weapons, and until now we believe that there might have some blast in it due to some carelessness" he said. "We are keeping all our options open."
Prime Minister Shehbaz Sharif initially called the blasts a "suicide attack", but later tweeted that that "the nature of the blast is being investigated".
Pakistani counter-terrorist forces maintain a strong presence in the valley, which has been prone to insurgency.
In 2012, militants also shot and wounded Nobel Laureate Malala Yousafzai in the valley.