The investigation was opened after the Colorado Department of Public Health and Environment received "numerous" complaints regarding "a ketamine administration in August 2019," according to a statement from the department.
Though McClain's death was not explicitly mentioned, it was on Aug. 24, 2019, when he was administered ketamine.
The Colorado Department of Public Health and Environment did not offer any specifics about the investigation, except to say it was ongoing.
McClain was walking home from buying iced tea at a corner store that evening when he was stopped by police, McClain's family attorney, Mari Newman, previously told ABC News.
Police received a call about him being "sketchy" because he was wearing a ski mask, though the caller noted that no weapons were involved and no one was in danger, according to a transcript of the call.
When police stopped McClain, he continued to walk despite being told to stop and a struggle ensued.
Officers placed him in a carotid control hold, which involves an officer placing his arm around a person's neck, restricting the flow of blood to the brain from the carotid arteries, police said.
Paramedics with the Aurora Fire Department also gave McClain ketamine, as per their department protocol used for "rapid tranquilization in order to minimize time struggling," according to officials. Paramedics also said that McClain possibly suffered from a condition called excited delirium.
The condition is typically associated with the use of drugs that alter dopamine processing, but is also notably brought up when a person dies in the custody of law enforcement, according to a public health report published in 2011.
The Adams County coroner said in McClain's autopsy report that there was a "therapeutic level" of ketamine in his system, though Newman called it an "excessive" dose.
"He weighed 140 pounds and the dosage he got was for somebody who was at least twice that weight," Newman said.
McClain, who had been throwing up, was put in an ambulance where he suffered cardiac arrest, according to police.
Though police said he regained consciousness and was being treated at a local hospital, he died several days later. His death received renewed national attention in the wake of the death of George Floyd, an unarmed Black man killed by Minneapolis police in May.
Newman told ABC News on Wednesday that McClain should not have been given any ketamine in the first place.
Newman said McClain was not presenting any signs of excited delirium, noting that he "was handcuffed and on the ground under the weight of multiple officers all bigger than him."
She also described the diagnosis of excited delirium as "suspect."
"The only time you even hear about excited delirium is in the context of law enforcement agents defending the amount of force they used against a person," Newman said. "You'll never hear of a person who is sitting in their living room and then experiences excited delirium."
She added that while the family is glad an investigation is now taking place, "it should have happened long ago."
Ketamine is most commonly used by medical practitioners and veterinarians as an anesthetic. However, between August 2017 and July 2018, 427 patients in Colorado received ketamine for agitation, according to state data obtained by The Denver Post. About 20% of those patients were later intubated in the hospital. No deaths have ever been reported to the state health department as a result of ketamine use, according to the the Post.
District Attorney Dave Young, who serves Adams and Broomfield counties, and has not pressed charges against the officers, said in an interview with ABC News' Linsey Davis in late June that he believes "a serious heart condition ... led to his death."
"My initial impression was that it was the ketamine that perhaps caused Mr. McClain's death," Young said. "And it wasn't until I received the forensic autopsy report that I learned that that in fact was not the cause of death."