A suicide bomber struck a mosque in a Pakistani tribal region during Friday prayers, killing at least 40 people and wounding 85 others.
The attack came during the holy month of Ramadan, a time of fasting, sharing and heightened community spirit for Muslims.
No group immediately claimed responsibility, but the Taliban and other Islamist militants have previously targeted mosques, especially if they believe enemies - such as army soldiers or anti-militant tribesmen - are using the facilities.
The mosque is in Ghundi, a village in the Khyber tribal region which has long been a base for Islamist militants.
Khyber is also a key region for the US and Nato, because a large portion of non-lethal supplies heading to US forces in Afghanistan passes through it.
Some 300 people had gathered for prayers in the Sunni mosque, and many were on their way out when the explosion occurred.
"All the evidence we have gathered confirms that it is a suicide attack," said an official.
Saleem Khan, 21, said people panicked after the blast, and that amid the smoke, cries and blood, several ran over him when he fell.
"Whoever did it in the holy month of Ramadan cannot be a Muslim," he said from a hospital bed in the main north-west city of Peshawar. "It is the cruellest thing any Muslim would do."
Also today, two US missiles struck a house in a tribal region that was once a Pakistani Taliban stronghold, killing four people, intelligence officials said.
The strike came as Pakistani-US relations are struggling since the unilateral American raid that killed al Qaida leader Osama bin Laden.
The two missiles hit a house in the South Waziristan tribal area, according to Pakistani intelligence officials.