New York (CNN) -- The third and final day of New York City's Electric Zoo music festival was canceled Sunday after two concert-goers died and at least four others were hospitalized due to drug-related causes, police said.
The cause of death for the two Electric Zoo attendees is believed to involve the drug MDMA, either in Ecstasy pills or in its "pure" powder or crystal form, known as molly.
Police identified the two victims as Jeffrey Russ, 23, and Olivia Rotondo, 20.
Four others remain hospitalized and critically ill, with the causes still under investigation, according to Sgt. Lee Jones, a New York Police Department spokesman.
The city recommended the electronic music festival be canceled after the recent events, and Electric Zoo's promoters, Made Event, agreed, Jones said.
In a statement posted Sunday on the event's homepage, Made Event said "the founders of Electric Zoo send our deepest condolences to the families of the two people who passed away this weekend. Because there is nothing more important to us than our patrons, we have decided in consultation with the New York City Parks Department that there will be no show today."
Social media postings regarding Electric Zoo's cancellation vary from sympathetic to downright infuriated.
General admission passes cost $179 per day for the festival, held at Randall's Island Park in the East River. The promoters' website said people who bought tickets for Sunday would get full refunds.
"Large bags," "bad attitudes" and "illegal substances" were among the 22 listed types of items prohibited at the venue, and everyone was searched upon entry, according to the website.
Founded in 2009, Electric Zoo features electronic dance music, with more than 110,000 people attending the festival in 2012. The three-day festival started on Friday and was scheduled to run through Sunday night.
What is MDMA?
MDMA was created in Germany in the early 1900s, according to the National Institute on Drug Abuse. Decades later, the drug found fans across the Atlantic.
During the 1970s some American psychiatrists felt they'd found in the drug a kind of "penicillin for the soul." The drug was said to allow for greater insights and better communication.
U.S. officials disagreed, and in the 1985 the Drug Enforcement Administration banned the substance as a Schedule I drug, meaning it had no proven therapeutic value. In the 1990s, MDMA gained a reputation for party drug at all-night warehouse parties known as raves. Emergency-room visits rose steadily from 421 in 1995 to a peak of 5,542 in 2001, according to DEA statistics. Questions were raised about safety and purity of Ecstasy.
That's when molly was born.
Molly -- short for "molecule" -- is touted as the pure form of MDMA, but a spokesman for the DEA says don't believe the hype. According to Rusty Payne, the agency sees MDMA from Asia, Canada and the Netherlands.
"You have no idea the lab environment these chemicals or substances were produced in," Payne said. "If they knew where things were produced, they might think twice."
In 2009, government data found 22,816 emergency-room visits due to MDMA, a 123% percent increase from 2005.
But molly has fans among some in the medical field. An article published last year in the Journal of Psychopharmacology found that the drug helped reduce the symptoms of PTSD for two-thirds of those enrolled in a study. Still, the sample size in that study was small, just 19 people. More studies are in the works.
Despite any potential for future uses, authorities warn that the drug is dangerous. After a giant New Year's Eve party in Los Angeles in 2010, one person died and 18 others were hospitalized for issues relating to MDMA use, according to the Centers for Disease Control and Prevention.
CNN's Chris Kokenes and Emma Lacey-Bordeaux contributed to this report.