The NBI has started sending subpoenas to witnesses, including foreign forensic experts, to establish claims that Villaruel and his accomplice Navy Lt. (sg) Ricardo Catchillar were murdered by policemen after they seized the control tower of the Ninoy Aquino International Airport (NAIA) on Nov. 8.
"We will establish if there was great danger when the two took over the tower and whether there was an attempt to peacefully settle the issue," an NBI agent told The STAR.
The agent said foreign experts are expected to share their findings on whether the seizure of the tower adversely affected operations of incoming international flights.
Airport authorities had earlier stressed that it was imperative for them to retake the control tower after Villaruel and Catchillar seized the facility in an alleged bid to expose corruption in government.
The family of the slain former ATO chief, however, claimed there was an "overkill" in the police operation.
The Villaruels sought the assistance of the NBI to disprove claims by airport and police authorities that the killings of Villaruel and Catchillar were justified.
The family pointed out that the duo were about to surrender when they were gunned down by policemen.
Lawyer Josue San Pedro, counsel of the Villaruel family, said they have asked the NBI to prove the claim that the former ATO chief was murdered.
"The captain was a victim of impulsive killing," San Pedro said.
"My father is not a criminal and not a terrorist and should have not died that way," said Villaruel’s daughter Mary Ann Timbol.
Timbol said the NBI advised them to secure authenticated copies of autopsy reports and post operations reports from the Philippine National Police (PNP) which initially conducted the investigation of the case.
San Pedro said they will help the NBI secure more witnesses to prove claims that the police used excessive force in retaking the airport control tower.
Malacañang earlier said the killing was a justified act, taking into consideration the circumstances surrounding the seizure of such a vital installation.
Even as security officials downplayed the tower siege as an isolated act, lawmakers pressed for an investigation of the incident.
President Arroyo then agreed to an independent NBI probe of the use of supposedly excessive force "to make sure that the result will be acceptable to all parties concerned."