Jonel Nuezca turned himself in on Sunday after he was accused of shooting his neighbour Sonya Gregario, 52, and her 25-year-old son Frank.
A video, shared on social media, shows him firing another bullet at Ms Gregario as she lies on the ground.
The killings have prompted an outcry and renewed calls for police reform.
Mr Nuezca, who was accompanied by his young daughter at the time, has not publicly commented on the incident. But local police chief Noriel Rombaoa told local broadcaster DZMM that he admitted to the killings.
The shooting took place in the city of Tarlac as Mr Nuezca, who was off duty, tried to arrest Initial media reports said a scuffle began after Mr Gregario set off an air cannon, made out of PVC pipe, which created a loud noise. A heated argument then ensued about the cannon and an ongoing land dispute between the two families.
As Ms Gregario wrapped her arms around her son to stop him being taken away, the video appears to show Mr Nuezca shooting her in the head, before doing the same to Frank.
Alyssa Calosing, who said she recorded the video, told DZMM that other witnesses said they saw Mr Nuezca beat Mr Gregario before she began She claimed that after shooting the pair at point blank range, Mr Nuezca took his daughter and "walked off like it was "I felt numb, like my soul left my body," said Ms Calosing. "Then when I felt something move, I began jumping in anger and crying."
Debold Sinas, chief of the Philippine National Police, told local media that Mr Nuezca had already faced five administrative charges, including two for murder, though both were dismissed due to a lack of evidence. Mr Nuezca was still allowed to carry a gun while off-duty.
Since the video emerged online, various hashtags have been trending on social media calling for justice.
Eduardo Año, the Philippines' interior minister, condemned the killings and stressed that the incident was isolated.
Earlier this year, a UN report found that more than 8,000 people had been killed by police since President Rodrigo Duterte took office in 2016.
The international body, along with human rights groups, has accused him of granting impunity to police during his war on illegal drugs.
Duterte's administration has denied accusations of wrongdoing, and the president remains popular in the country.
HOW COME WHEN PEOPLE NEED HELP I ALWAYS SEE FILIPINOS BOOSTING AND SHARING THERE VIDEOS BUT WHEN ITS OUR COUNTRY (Philippines) PEOPLE STAU QUIET ABOUT IT I DONT GIVE A FUCK IF YOU HAVE 10 OR 10000 FOLLOWERS WE NEED JUSTICE FOR NANAY SONYA AND FRANK SONYA IF YOU DONT CARE ABOUT THIS YOU CAN LEAVE AND NEVER READ MY STORIES AGAIN
#Stopthekillingsph
