She didn't know anything serious was happening until she was told by another hotel guest to evacuate immediately. With no time to grab a jacket, she fled to the nearest stairwell—and that's when the toxic odor hit her. "It smelled, for all the world, like the worst Pool Shock you've ever been around," she reminisces, referring to a type of pool cleaning chemical. "Like it was eye-stingingly bad, even outside the hotel."

It was after midnight on a Sunday when the fire alarm went ringing through the halls of Hyatt Regency hotel in Rosemont, Illinois. Phaedra Lewis had planned on ignoring it, figuring it had been innocently triggered by the smoke of a cigarette. Besides, it was cold outside and she was already in her pajamas, having stopped by a friend's room to hang out before going to bed.

When Lewis appeared briefly in the background of a national television newscast, it triggered panic among her family. More than 600 miles away in Asheville, North Carolina, Lewis's mother was woken from her sleep in a nursing home and informed that her daughter had been involved in a terrorist attack. Lewis, who lives in a suburb near Chicago, assured her mother she was fine. Her cell phone blew up with texts from co-workers who asked if she'd been hurt. Their second question: Are you at a furry convention?

It didn't take authorities long to confirm what many convention attendees had intuitively suspected: The intense fumes they'd smelled were the result of chlorine, the oxidizing chemical commonly used as a cleaning agent in swimming pools. The gas can be toxic when leaked into the atmosphere, causing respiratory problems and irritation of the eyes. Nineteen people were sent to the hospital as a result.

The Rosemont police and fire departments rushed to the scene around 12:45 AM on December 7, 2014. But it wasn't just local law enforcement that stormed the hotel. There were throngs of reporters with bright cameras, hazardous materials technicians wearing space-like suits, and later, detectives from FBI Chicago's counter-terrorism and weapons of mass destruction unit.

The Rosemont Police Department launched a criminal investigation into the spread of the chlorine gas, enlisting the help of federal investigators. But more than a year later, no charges have been filed and neither agency has made an arrest. Today, the source of the chlorine gas that sickened 19 people and catapulted furries into the media spotlight remains a mystery. Convention attendees still reminisce about standing in the cold together until the wee hours of the morning, and while some laugh it off as an unfortunate one-time prank, many others are still searching for answers.

The incident became a national news sensation not only because authorities deemed it a deliberate, criminal act, but also because it occurred during Midwest FurFest , the second-largest furry convention in the country. The annual gathering brings together more than 4,000 people from all over the world, many of whom engage in role playing as anthropomorphic animals, sometimes while dressed in head-to-toe fur suits.

Special Agent Garrett Croon, a media coordinator for the FBI's Chicago Division, however, said that while the Rosemont Police Department may have closed its case, it's not uncommon to reopen an investigation if either department were to get a lead. "It's always ongoing because whether a year from now or three years from now, evidence is developed or tips are called in or somebody comes to the FBI and informs us, 'Hey, I know who the bad guy is,'" he told me over the phone. "Well, if it's not past the statute of limitations, the FBI reserves the right with the US Attorney's Office or the State's Attorney's Office to prosecute the case." He said they still consider it a criminal investigation.

An incident report filed by the Rosemont Police Department shows the case was logged into the system on December 8, 2014, assigned to a detective on December 29 of that year, and closed on July 29, 2015. But the reports reveal little about what took place after December 2014—let alone seven months later. The last page of the report sent to VICE by the Rosemont Police Department shows that the FBI had emailed the department its set of reports relating to the Midwest FurFest. The FBI did not respond to a request for the documents.

"Who would've done it? Was it a furry from the inside who was looking for attention in this sort of messed up way?" asked Tommy Bruce, a Maryland-based photographer who attended Midwest FurFest in 2014 and has been documenting furry conventions all over the country for the last six years.

Detectives from the Rosemont Police Department declined to comment directly on the case, but reports they provided to VICE show that in the days following the incident, officers interviewed at least 30 hotel guests, more than 19 hotel employees, and a number of hospital workers, taxi drivers, and staff employed at local pool and hardware supply stores that sell chlorine. While officers investigated the whereabouts of several individuals, it's unclear how many were considered suspects and during what time.

In another photo, a shirtless man wearing a bear head and a leather harness strapped around his chest raises a fist to the sky, as if in protest—or joy, or maybe rage. But not every scene was quite as pleasant: One image shows a woman on a gurney being wheeled into the back of an ambulance; another shows a man gasping for breath as he clings to someone in a white fur suit for support.

Photos Bruce captured the night of the evacuation—all tinted red and blue from the flashing lights of nearby cop cars—depict scenes of chaos, panic, compassion, friendship, and then boredom as crowds waited in the convention center across the street for hours before they could enter the hotel again. In one image, a person dressed as what appears to be a large black and white skunk wraps white fuzzy mittens around a friend's shoulders; the friend cradles his mascot-like lion head under his arms.

Though Midwest FurFest panels, dances, and exhibits were scheduled in ballrooms and meeting halls on the first three floors of the hotel, attendees booked a majority of the 1,000-plus guest rooms throughout all ten floors, meaning furries were spread throughout the hotel that night.

When first responders arrived at the scene that night, they used a chlorine meter to lead them to the source of the noxious odor. Donning self-contained breathing apparatuses—or large face-covering masks attached to an air tank that's worn like a backpack—the firefighters headed to the ninth floor of the Hyatt Regency, where the meter recorded a gas level of 1.4 parts per million. That's about the rate at which humans will generally start to experience mild irritation from chlorine, and can typically only tolerate it for about an hour or so, according to the National Center for Biotechnology Information (NCBI).

But hotel guests complaining about having itchy, red eyes and trouble breathing had reported the odor long before emergency responders encountered it, says Lewis, who had been on staff at Midwest FurFest at the time. "But it was at night during the convention, many of them had had a few drinks, so our medical [team] just assumed, 'Oh well, somebody spilled something on the stairs, maybe a maid did it or something,'" she said. "It was only fairly late in the evening that it really became clear, somebody had done something deliberately."

By the time firefighters reached the stairwell of the west wing, the gas level had soared beyond 60 parts per million—double the rate at which people exposed to it immediately start to feel chest pain and shortness of breath—exceeding the meter's maximum reading. Humans who inhale that level of chlorine in the atmosphere risk contracting toxic pneumonitis or acute pulmonary edema, which can develop into respiratory disease, according to the NCBI.

"Outside of initial first responders and assisting in the evacuation of our attendees and staff, Midwest FurFest relinquished complete control of the onsite emergency response and the subsequent criminal investigation," the convention wrote in a statement published last November. "The furry community has been exceptionally supportive of our convention in the wake of this criminal act and our resilient staff and remarkably understanding and sympathetic attendees helped us finish the weekend on many positive notes." Matt Berger, the convention's director of programming and marketing, declined to comment further, citing the ongoing investigation.

Firefighters spotted the evidence in a stairwell landing between the ninth and tenth floors: a white powdery substance and the broken glass remains of what appeared to have once been a mason jar. The firefighters retreated from the stairwell and requested back-up assistance once they noticed a yellow and green liquid running down the walls. When the hazmat technicians arrived, they swabbed eight samples—both of the wet liquid and the dry powder—from four different stain patterns on the walls and the landing of the stairwell. The samples were then packed in absorbent pads in a steel drum, but the tests later conducted turned up inconclusive due to a faulty instrument, according to the police reports. Investigators had already confirmed the heightened levels of chlorine gas with the chlorine meter. It's unclear whether there were substances other than chlorine present.

"Twitter was just like blown up for the entire night with people at the convention," recalls Bruce, explaining that furries commonly use the social network to follow stories and updates from a convention—especially if they can't attend in person. "It was interesting to be a part of that experience within the furry community where there is this sort of internet megaphone–like system throughout the whole community, throughout the whole [chlorine incident], and there's so much ability to communicate ideas rapidly."

Midwest FurFest sent out a series of tweets throughout the evacuation to keep attendees informed of the situation—but the tweets provided little clarity, stressing how little organizers knew about the nature of the emergency. In the absence of information, rumors quickly circulated, becoming magnified and multiplied through social media.

After all the hotel guests—including those who had nothing to do with Midwest FurFest—got herded into the nearby Donald E. Stephens Convention Center about an hour after the evacuation began, social media served another practical purpose: locating those who had been separated during the evacuation. "[People] were holding their cell phones up in the air in the convention center and taking panoramic pictures and then posting them to Twitter so that you could make sure your friends were OK," remembers Lewis.

Some of the early speculation included one theory that chlorine had leaked from a hotel swimming pool or a storage area—or that maybe a ceiling pipe or an air conditioner had sprung a leak, spewing nasty chemicals out into the atmosphere. But the Hyatt Regency O'Hare didn't have a swimming pool—and why would chlorine be carried through pipes, anyway? Others posited that maybe things got out of hand during a domestic dispute; or that a kid's science experiment had exploded; or that a hotel guest had decided to clean rubber work equipment with chlorine products.

But being holed up in the convention center wasn't all doom and gloom. After all, many people had been coming from parties and raves that typically last until all hours of the night on the Saturday of the weekend-long convention. Some were intoxicated or chemically altered; and some were in various states of undress, from fetish wear to pajamas to full-on fur suits.

"One guy seemed to be trying to start a revolution," said Pieter Van Hiel, a Hamilton, Ontario–based science fiction writer known for authoring a series of role-playing games set in 17th-century Japan. "He was standing there and in a very loud dramatic voice he kept starting to deliver an inspiring speech but would get like three or four words in and forget what he was saying and start again and a bunch of people told him to sit down."

Coincidentally, the convention center had been hosting another kind of furry convention earlier that day—only at that one, the animals on display were real. The cages and kennels strewn about suggested that a dog training show had taken place in the massive auditorium, and some of the dogs had been left in crates overnight, their barking and yapping audible, according to Bruce.