COTABATO CITY, Philippines ? (UPDATE) A key witness in the trial of a powerful Muslim clan accused of orchestrating the worst political massacre in the Philippines has been shot dead, a police official has confirmed.

But Senior Superintendent Alex Lineses, Maguindanao police chief, said Thursday that they were still investigating the killing of Suwaib Upham in Parang town around 8 p.m. of June 14.

Upham claimed to have taken part in the November killings of 57 people in a crime allegedly planned by his former employers, the Ampatuan clan.

Lawyer Harry Roque disclosed to Agence France-Presse earlier in the day that Upham, whom the prosecutor described as ?our strongest witness,? was killed and that his death could potentially weaken the case against the Ampatuans.

Sources, who refused to be named because they feared for their safety, said Upham has been moving from one place to another to avoid harm after agreeing to testify against the Ampatuan family.

His death was recorded in the town?s police blotter as just another case of killing and there was no mention of his being a witness in the massacre case.

?The gunman remained unidentified and the motive was still unclear. We are still determining the real motive of the attack,? Lineses told reporters.

US-based Human Rights Watch also said the killing raised doubts about the government's resolve in seeing justice done in the case.

"Massacre witnesses are dying while the government sits on its hands," the group's Asia director Elaine Pearson said in a statement. "This sends the worst possible message to other witnesses thinking of coming forward."

Upham?s death comes two months after an uncle of another witness was also shot and killed, in what authorities said was part of a plan to intimidate those speaking out against the Ampatuan clan.

The clan had ruled Maguindanao with brutal efficiency for a decade prior to the massacre. They also enjoyed political ties with outgoing President Gloria Macpagal-Arroyo, who used the family's huge private army as a force against separatist rebels.

Six clan members are among 196 people charged over the murders, allegedly carried out to prevent a member of a rival clan from running as governor of the province.