DAGUPAN CITY, Philippines -- A policeman has been arrested in connection with the killing of Agno Mayor Arthur Cabantac, a Pangasinan police official said on Tuesday.

But Senior Superintendent Isagani Nerez, provincial police director, withheld the identity of the policeman who is now under investigation.

"The policeman was seen by witnesses carrying an M-16 rifle a few hours after the shooting," Nerez said in an interview over a local radio station.

He said the policeman has undergone a paraffin test and his firearm has been submitted for ballistic testing.

Cabantac, 55, was shot and killed on August 2 while playing mahjong in a neighbor's house in Agno town.

Nerez said Cabantac had earlier requested the relief of the arrested policeman because he was allegedly "abusive."

The suspect, Nerez said, was a former security aide of former mayor Leticia Rosete, who Cabantac defeated by 68 votes in the 2007 elections.

On Monday, Pangasinan Governor Amado Espino Jr. and the League of Municipalities of the Philippines put up a P500,000 reward for information that would lead to the arrest of Cabantac's killer or killers.