One Wisconsin family is searching for answers about what really happened the day their 20-year-old daughter died during what should have been a relaxing family vacation. Abbey Conner’s death was ruled accidental after she drowned in chest-high water at the Iberostar Paraiso Resort in Playa Del Carmen, Mexico—but her family isn’t satisfied with that explanation.

“Someone needs to be held responsible for this,” her father, Bill Conner, told ABC News Monday. “It doesn’t make any sense.”

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Abbey and her brother, Austin Conner, 23, were drinking at the resort before they went for a swim. They were set to meet their mother and stepfather for dinner shortly after. After both children didn’t show up, their parents were informed they had been found unconscious in the hotel pool and had been taken to a nearby hospital.

Austin had a concussion while Abbey had a broken collar bone. Austin was able to recover — Abbey was not. The University of Wisconsin-Whitewater student was unresponsive and in a coma. Her life support was withdrawn a few days later.

Mexican officials ruled her death an “accidental drowning.”

“She was healthy,” Bill Conner told ABC News. “But for some reason, she drowns in a pool. Waist deep. It’s unbelievable.”

The last thing Austin remembered was he and his sister taking a shot of an unidentified alcohol at a bar with what he thought was a friendly couple. His next memory was waking up inside an ambulance. And while their blood alcohol was found to be about .25, Austin said there’s no way that could account for what happened.

“I’ve been in college for five years and had my fair share of drinks before,” he told the Milwaukee Journal Sentinel last week. “No way in hell I’m putting my face down in a pool and going to sleep.”

The investigation by Mexican authorities proved to be limited. The police department told an attorney for the family that it had interviewed just three of the hotel’s employees. And while a blood test showed no trace of opioids, benzodiazepines (a type of date rape drug), or cocaine, the test didn’t test for all types of drugs.

“It was all too convenient,” the family’s attorney, Florentino Ramirez, said of the police report. “If it was an accident, where was everybody? It just doesn’t make sense. There are too many open ends.”

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Bill Conner said he was sure somebody slipped them some type of drug at the resort. In addition, two blog posts uncovered by the Journal-Sentinel revealed reports of similar incidents recorded by other travelers at the Iberostar’s sister resort. The United States Department of State told ABC News they were “aware of the case” and were working to provide consular assistance but did not comment further on the case.

“We’re lucky,” Bill Conner said, “we didn’t lose both of our kids.”