Provincial police chief Senior Superintendent Arturo Cacdac told reporters that investigators are sifting for clues among the court cases handled by Uson.
Police suspect that the killers were sent by someone who lost a case before Uson’s court. Cacdac said they are following other leads but refused to elaborate.
A local crime group, known as the "Aguila Gang" with links to local politicians, may also be involved, police earlier said. Some of the gang’s members were sent to jail in the past by Uson.
Regional police commander Chief Superintendent Arturo Lomibao, who formed "Task Force Uson" to speed up the investigation, earlier said they have witnesses who may help them solve the case in a week.
He said several witnesses volunteered to give information.
Uson was ambushed late Friday afternoon while driving through Asingan town by four motorcycle-riding gunmen.
Family members said Uson told them he received constant death threats and refused advice to hire a bodyguard, preferring instead to carry a gun.