Philippine police officials will investigate if other members of the Ampatuan clan are involved in the recent Maguindanao massacre that claimed 57 lives, said a high-rank police official Thursday.

Police Director for Operations Andres Caro II said in Thursday' s press briefing that initial findings showed that Datu Unsay Mayor Andal Ampatuan Jr. is involved in the killings.

He said other members of the Ampatuan family, including patriarch and former Maguindanao Governor Andal Ampatuan Sr., may be invited for questioning. But he noted the investigations are still far from over.

Caro said it took a while before Andal Jr. was identified and charged as a suspect in the incident because there was no immediate evidence linking him to the massacre.

Deputy Presidential Spokesperson Lorelei Fajardo said due process must still be observed adding that while the names of the Ampatuans had came up after the incident, there has to be proof or evidence to back up the allegations.

Caro said all 374 Special Cafgu Active Auxiliary (SCAA) members and four policemen that were detailed in Maguindanao are being investigated for their possible involvement in the massacre.

He said both the police and military also had to work quietly so as not to make those involved scamper away.

Ampatuan was tagged as the mastermind behind the gruesome killing of 57 people which included members of the media and the supporters, staff and family members of Buluan Vice Mayor Esmael Mangudadatu. The Mangudadatu and Ampatuan families are political rivals.

A state of emergency was declared in Maguindanao, while a Day of Mourning was also observed Thursday following Monday's massacre.

Flags were at half mast in government offices, while journalists covering the president were either in black or wearing black armbands in honor of those who perished.