Among those killed were Macmod Mending, National Irrigation Administration (NIA) regional director for Central Mindanao; engineer Omarkaes Mamalak, Maguindanao provincial irrigation officer, and Ustadz Ismael Datu Kali, an imam presiding over the prayers.
The fourth fatality remains unidentified.
Mending died of his wounds on the way to Notre Dame Hospital in Cotabato City, while Mamalak was declared dead on arrival at Midsayap Community Doctor’s Hospital.
No one has claimed responsibility for the bombing.
Most of the wounded are employees of NIA, which owns the compound in Barangay Villarica, where the mosque is located.
One of those seriously wounded was engineer Alimudin Dilangalen, 50, a supervisor at NIA’s engineering and equipment office in Northern Mindanao.
The wounded were taken to nearby hospitals.
Dr. Gemma Libarios, director of Midsayap Community Doctor’s Hospital, said nine of them sustained minor wounds but have to be confined so doctors could monitor their conditions within the next 24 hours.
Witnesses said they heard two loud, almost simultaneous explosions, and rushed to the mosque, where they saw the worshippers running in different directions.
Some were lying bloodied on the floor, they added.
Witnesses said they saw two men fleeing from the scene on board a motorcycle after the explosion.
Midsayap police commander Chief Inspector Eduardo Marquez said the safety pins of the two fragmentation grenades were recovered about one and a half meters from the door of the mosque.
"We’re facing a blank wall (in the investigation)," he said.
Marquez said the attack could be linked to "an internal problem involving NIA officials" in North Cotabato.
He did not elaborate.
The suspect fled as the grenades went off at 12:20 p.m., he added.
Lt. Col. Daniel Lucero, Armed Forces public relations chief, said "an undetermined number of men" carried out the attack but could provide no further details.
The local police also said they have no immediate suspects or motive for the attack.
Quoting field reports, Datu Zamzamin Ampatuan executive director of the Office of Muslim Affairs, said the killers, who had taken part in prayers, suddenly stood up and lobbed the grenades on the worshippers.
"We condemn the attack and we call authorities to conduct objective investigation, and we also appeal to the public to stop speculating on the motive of the attack," he said.
The attack was an offshoot of "an internal struggle," Ampatuan said without elaborating.
In Davao City, Eid Kabalu, Moro Islamic Liberation Front spokesman, said the MILF had nothing to do with the grenade attack.
"We have nothing to do with what is happening with the bombings elsewhere, as we have said before we have nothing to do with them," he said.
Authorities said Mending and Mamalak could have been the target of the attack, following the reported lay-off of some NIA employees in Central Mindanao.
Meanwhile, the Cotabato chapter of the Philippine National Red Cross said it had sent 30 units of blood to various hospitals in North Cotabato and nearby areas for use of the victims of the grenade attack. — With reports from Roel Pareño, Jaime Laude, Perseus Echeminada, Edith Regalado, AP, AFP