MANILA -- Datu Unsay Mayor Andal Ampatuan Jr., the prime suspect in the massacre last Monday of 57 people in Maguindanao, has tested negative for illegal drugs, according to the National Bureau of Investigation (NBI).

An NBI source said there was no trace of illegal substances in Ampatuan’s urine sample.

The source said Ampatuan only tested positive for the use of benzodiazepine, a sleep inducing drug.

The massacre suspect admitted to investigators that he has been taking a drug called Rivotril.

The NBI source, however, said that the drug test does not exactly clear Ampatuan from drug use as some illegal substances can be cleared from urine in just 48 hours.

Ampatuan was picked up by authorities Thursday morning, three days after the country's worst ever election-related violent incident.

The country’s top anti-narcotics chief said they have received information that drugs were also involved in the massacre.

Majority of the victims were members and supporters of the Mangudadatu clan, including the wife of Buluan Vice-Mayor Esmael Mangudadatu, who is opposing the Ampatuans in the gubernatorial race in the province.

“’Yung patayin mo, abnormal na. Imutilate mo pa, i-rape mo pa… ang description nga [sa suspects] eh hindi na animal, demonyo na,” said Dionisio Santiago, Philippine Drug Enforcement Agency (PDEA) chief.

The drug test was among the medical procedures which Ampatuan was made to take after he was brought to the NBI custodial center.

“Everything was checked lalo na ['yung] body injuries... we conducted a thorough physical examination, kung meron man siyang injuries sustained elsewhere. So far wala naman,” said Dr. Florencio Arizala Jr., NBI Medico-Legal Division chief.

Arizala said that except for having asthma, Ampatuan is relatively healthy.

Security at NBI beefed up

NBI Director Nestor Mantaring said they have beefed up security inside the bureau compound to protect Ampatuan from attacks.

An ABS-CBN News report said four armed men on board two vehicles tried to enter the NBI compound on Thursday night after Ampatuan Jr.'s arrival from Maguindanao province.

The NBI said the four men's firearms were licensed, but they did not allow them to enter the compound.

Meanwhile, a certain Inspector Musa Umpan was disarmed by NBI agents after he also tried to enter the compound before Ampatuan Jr.'s arrival from Maguindanao.

Umpan claimed he was an aide of Maguindanao provincial administrator Nori Unas.

The two incidents have prompted the NBI to deploy at least 30 heavily-armed agents inside the compound.

"High risk prisoner ito sapagkat alam natin ang nangyari kung saan siya ay naakusahan," said Mantaring. -- With a report from Maan Macapagal, ABS-CBN News