University of Michigan student Lucas Mattson was last seen at 1 a.m. on Jan. 23 amid cold temperatures and was found dead the next day

Mattson’s autopsy report stated the cause of his death as hypothermia, and described the manner of death as accidental

An attorney for the Mattson family tells PEOPLE that they are preparing litigation against a fraternity whose party Mattson attended prior to his death

Autopsy results for a 19-year-old University of Michigan student who was found dead after he went missing during extreme weather conditions indicate that he was legally intoxicated.

The body of Lucas Mattson was discovered at 12:05 p.m. local time on Jan. 24 and recovered in the 1900 block of Cambridge Road, the Ann Arbor Police Department (AAPD) said in a statement.

Mattson was last seen at 1 a.m. on Jan. 23. According to the AAPD, the student was "walking alone ... without a coat” before he was later reported missing.

“The nearly 20-hour search effort to locate him took place in extreme cold conditions and included officers from AAPD and University of Michigan Division of Public Safety and Security, as well as the University of Michigan Police Department Drone Unit,” the AAPD said in its statement.

Mattson’s autopsy report, obtained by PEOPLE on Thursday, March 19, stated the cause of his death as hypothermia, and described the manner of death as accidental.

The report also indicated that Mattson’s blood alcohol content was 0.156% — nearly twice the 0.08% legal driving limit for those over the age of 21 in Michigan, as noted by The Michigan Daily, the university's independent student newspaper.

Dr. Randy Tashjian, the pathologist who handled the autopsy, wrote that acute ethanol intoxication was a contributing factor in Mattson’s death.

The findings also stated that there was “no evidence of significant acute or recent physical trauma” on his body.

The pathologist wrote in his findings that Mattson was found dead outdoors after being seen leaving a nearby house party in the early morning hours of Jan. 23.

A Jan. 26 message to the university from the school’s interim president, Domenico Grasso, said that Mattson attended a fraternity house party “as guest” on Jan. 23, adding that the student was not a member or a pledge of the fraternity. Mattson's body was later found just blocks from the fraternity house.

Robert Raitt, an attorney who represents Mattson's family, tells PEOPLE that the autopsy report was not surprising.

"We knew that he was drunk," Raitt says. "We knew that the fraternity provided him all of the liquor that he drank. We talked to a fraternity brother, a good friend of his that invited him to the fraternity [and] told us what we needed to know very early on."

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"The main thing we were looking for was to see that there was nothing else in his system that could've played a role, and there wasn't," he adds.

Raitt says that Mattson was friends with a couple of the fraternity brothers who invited him to the party that evening.

"He was celebrating a good interview he had with an Alaskan company to do a summer internship with an engineering firm," he says.

The University of Michigan Delta Chi fraternity chapter was placed on interim suspension on Jan. 25, according to the university.

Raitt also says that he has reached out to the local chapter and national headquarters of the fraternity, as well as the university. He adds that they are preparing litigation against the fraternity but says he has not yet filed a complaint in court.

The University of Michigan did not immediately respond to PEOPLE's request for comment.

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