At least three people have been killed and around 20 injured in a grenade attack on the southern Philippine island of Mindanao, police said.
Two hand grenades were thrown into a packed mosque during noontime prayers, said Chief Inspector Eduardo Marquez.
Midsayap, which is mainly Christian but has a sizable Muslim minority, has been hit by bomb blasts before.
But police said the likely spark for the attacks was local politics rather than religion.
The incident happened inside the compound of the government's National Irrigation Administration (NIA).
"The angle we are looking at is this is a struggle for power within the NIA. There is an internal struggle as to who would head the office," provincial governor Emmanuel Pinol told Reuters news agency.
Among the dead are Macmud Mending, the irrigation agency's regional director, and Ismael Datu Kali, the Islamic preacher presiding over the prayers.
Most of the casualties are thought to be government employees.
So far no one has claimed responsibility for the attack.
"We are still conducting an investigation," said Mr Marquez. "I believe it was done by only one person because a witness said so."
At the time of the blast, witnesses described hearing two loud, almost simultaneous, explosions.
Rushing to the mosque, they saw worshippers scattering in different directions.
Culture of violence
The Philippines is awash with weapons, and it is not unknown for Filipinos to try to settle seemingly minor private disputes by throwing hand grenades at their rivals, the BBC correspondent in Manila, John McLean, says.
But Mindanao island is also mired in factional fighting, with Islamic rebels being blamed for a string of bomb attacks in the region, including some earlier this year.
Our correspondent says the targets of these attacks are not usually other Muslims.
In years gone by, Christian vigilante groups have attacked Muslims, but usually in retaliation for attacks on Christians.