City Police Director Manuel Barcena said the explosion occurred in a vacant lot owned by Mateo and Wilson Velasco.
Barcena said conflict over ownership of the land could be the motive since the place of the incident is a contested lot.
The police are now trying to determine who the claimants are, Barcena said.
He said one of two unidentified persons, riding tandem in a motorcycle were the ones who hurled the grenade.
The second incident took place at dawn Tuesday in Alvarez Drive, Barangay Mampang, seven kilometers east of this city.
Barcena said one of the three unidentified men also riding tandem in a motorcycle were the ones responsible over the incident.
The explosion partially destroyed a community store owned by Mikih Anga, a retired soldier.
The police believed the incident stemmed from a personal grudge. But Anga denied saying he does not have any enemy for the past 30 years while he was in the military service.
Barcena said MK2 fragmentation grenades were used on both incidents. No one was hurt on both incidents.
