A 45-year-old man who died following an encounter with Fairfax County police after wandering away from his caretakers Wednesday was “severely autistic” and could not speak, his brother said Thursday.

Paul Gianelos, of Annandale, Va., had no history of violence, his brother Jim Gianelos said. About six months ago, Paul ­Gianelos wandered away from his caretakers but was returned by police without incident, his brother said.

Jim Gianelos said he was waiting for autopsy results before commenting on whether he thought officers handled the situation properly. He said the autopsy could be finished as soon as Thursday.

“It’s too hard to speculate on what happened at this point,” Gianelos said. “We are waiting for all the facts to come out.”

The incident began shortly before 1 p.m. Wednesday, when police were called to Roundtree Park in Falls Church for a report of a “critical missing man,” police said. Paul Gianelos had walked away from a lunch at the park organized by a group that helps people with disabilities.

Police said an officer found Gianelos about a mile away on a commercial strip on Annandale Road shortly after 1 p.m. The longtime officer, who is trained in dealing with people with cognitive disabilities and has been on the force for more than two decades, attempted to engage Gianelos in conversation, police said. Gianelos could understand speech.

The officer also called ­Gianelos’s caregiver and asked that the caregiver reunite with him, police said.

For unknown reasons, police said, Gianelos became combative with the officer and a struggle ensued.

The officer requested assistance, and three additional officers came to the scene. Police said Gianelos remained combative and was taken to the ground and handcuffed. He had a minor scrape on his forehead and an ambulance was called at 1:06 p.m., police said.

Fairfax County Police Chief Edwin C. Roessler Jr. described Gianelos as 5-foot-9 and 280 pounds, and he said he did not think officers used weapons while taking Gianelos into custody. He said he thought ­Gianelos was unarmed.

Police said Gianelos was alert and breathing when an ambulance was called; Roessler said Gianelos collapsed when medics arrived and that he went into cardiac arrest.

Medics began performing CPR at 1:17 p.m., and Gianelos was taken to Inova Fairfax Hospital, where he died shortly after arriving.

The death is being investigated by the police department’s major crimes division and internal affairs. Roessler said he planned to release the names of the officers involved and additional details about the circumstances of Gianelos's death as they become available.

"From all indications, the goal was to stop aggressive action and get help," Roessler said. "I truly believe the compassion was there, but it's a horrible tragedy."

Roger Deshaies, president of St. John’s Community Services, said Gianelos was in the care of the group when he wandered off.

For more than 20 years, ­Gianelos had been part of a program that helps people with disabilities become more independent by orienting them to services in the community and giving them social experiences, Deshaies said.

He confirmed Gianelos left while members of the program were eating lunch at Roundtree Park.

St. John’s is launching an internal investigation into how Gianelos walked off and is hiring an independent investigator, ­Deshaies said. He said the probe was just getting started, so he didn’t yet know how Gianelos left the group, how long before he was reported missing or whether he had wandered off before.

“This has been a truly devastating event, not only for his family but for our staff,” Deshaies said. “We’re all struggling to understand what happened.”

He said it was too soon to comment on how the police handled Gianelos, but he said that, historically, his clients had positive interactions with authorities.

Ari Ne’eman, president of the Autistic Self Advocacy Network, said his group has had continuing concerns about interactions between the police and those with disabilities, after a number of high-profile incidents in recent years.

In 2015, an autistic sixth-grader in Virginia was charged with a felony after attempting to get away from a school resource officer. The charges were later dismissed.

In 2013, a man with Down syndrome died in Frederick County, Md., after off-duty sheriff’s deputies forcibly removed him from a movie theater.

Ne’eman said he wanted a thorough investigation of ­Gianelos’s case.

“It’s clear something went very wrong here,” Ne’eman said. “We want a full and comprehensive investigation of the police conduct and why Mr. Gianelos wandered.”