Metro Manila (CNN Philippines) — Authorities have opened an investigation into the deaths of five people, including an American, who attended a weekend open-air concert in Pasay City.

The National Bureau of Investigation (NBI) said an autopsy indicated at least two of the victims died of a drug overdose.

The National Capital Region Police Office (NCRPO) on Sunday (May 22) confirmed the five attended the 12-hour Closeup Forever Summer Concert, which started Saturday (May 21) afternoon.

The Pasay City police said in a report that about 14,000 people attended the concert at the Mall of Asia parking area.

The Closeup 'Forever Summer' concert tragedy: What we know so far

The report said the five people, including American Eric Anthony Miller, 33, were “rushed separately” to hospitals “after they experienced difficulty breathing and eventually collapsed” during the concert.

Ariel Leal, 22, Lance Garcia, 36, and Ken Migawa, 18, were brought to the San Juan De Dios Hospital in Pasay, the report said. Garcia was pronounced dead on arrival at 3:37 a.m. Sunday (May 22) by attending physician Dr. Amado Flores.

Bianca Fontejon, 18, and Miller were brought around 2 a.m. to the Manila Doctors Hospital where they died, according to attending physician Dr. Roberto Ruiz.

Cause of death: Illegal drugs, heat stroke, or poisoning

In an interview with CNN Philippines, NBI medico legal division chief Atty. Floresto Arizala confirmed that autopsies performed on Fontejon and Garcia showed they died of a “massive heart attack.”

NBI Chemical Forensics Division Executive Officer Felicisima Francisco said the two had "very high blood pressure" which caused ruptures to their hearts.

“We are trying to finish our examination on the submitted specimen to determine really what are the drugs that may be have been taken by the persons who died,” Francisco said.

The NBI is conducting a toxicology examination on tissue samples taken from the victims to identify the substance that could have caused the heart attacks. Investigators are verifying reports that a potent drug called "green amore" was distributed among some of the concert-goers, Arizala said.

Joel Tovera, director of the NBI Anti-Illegal Drugs Division, said the indicators of a drug overdose were seen in the two victims — heart attack, kidney failure, dehydration.

Pasay City police chief inspector Rolando Baula said, they are looking at three possible causes of death — effects of illegal drugs, heat stroke, and poisoning.

Organizers saddened by the incident

In a statement released Sunday on its official Facebook page, Closeup extended its "deepest sympathies to the families of those who've passed away" and vowed to support "those needing medical assistance so they may fully recover."

Closeup said it also followed strict protocols to provide medical assistance, as the victims were immediately brought to the nearest hospitals.

The company said it regrets the incident despite the "very stringent measures and precautions" put in place to protect the safety and security of concert-goers.

Closeup said it will cooperate in the investigation.

SM Supermalls President Annie Garcia, in a statement released on Sunday, they will assist "the event organizers, Closeup Philippines, its agencies, and the authorities in the investigation."

There was no immediate word on the cause of death of Leal, Migawa and Miller. The family of Leal has refused an autopsy and there was no immediate permission from the U.S. embassy to conduct an autopsy on Miller.

Family asks for privacy

The family of 18-year-old Fontejon refused to make any statement to the media.

La Funeraria Paz-Parañaque manager Andy Akailar said her parents gave instructions not to allow media to cover her wake.

"Humihingi sila ng privacy… Talagang masakit para sa isang magulang na makita nilang ganoon ang sitwasyon ng anak nila," Akailar said.

[Translation: They are asking for privacy ... It is really painful for parents to see this happen to their child.]

House probe sought

Kabataan Partylist Rep. Terry Ridon on Monday (May 23) called for a congressional investigation into the deaths.

Ridon filed House Resolution No. 2704 urging the committees on dangerous drugs and youth and sports to conduct the probe.

The lawmaker cited reports saying the drinks distributed at the event were laced with a lethal mix of ecstasy and shabu known as "green amore."

“There are questions that investigators need to answer: If dangerous drugs were indeed available in the party venue, how were they able to slip in such substances? Are the concessionaires inside the venue involved? Clearly, the ‘stringent security measures’ or ‘precautions’ did not deter the proliferation of said dangerous substances. And surely what had transpired in the concert is not an isolated case,” Ridon said.

The lawmaker said they need to craft "legislation that balances anti-drug policy and public safety."

“While we do not condone the use of dangerous drugs in any event whatsoever, there are certain finer details that need to be considered in enacting a new law against drug proliferation,” Ridon added.

CNN Philippines correspondents Gerg Cahiles, Selina Dagdag, Joyce Ilas, Pia Bonalos, Isabella Montano, and Menchu Macapagal contributed to this story.