Nick Reiner's Hotel Room Full of Blood After Rob and Michele Murders

Nick Reiner, who is under arrest for the murder of Rob and Michele Reiner, checked into a Santa Monica hotel early Sunday morning, and left a trail of blood behind ... sources with direct knowledge tell TMZ.

Nick checked into The Pierside Santa Monica around 4 AM Sunday, using his credit card. His check-in came hours after he got into a heated argument with his father at Conan O'Brien's Christmas party.

Eyewitnesses who saw Nick check in say he seemed "tweaked out," but there were no visible signs he had been in a violent confrontation -- we're told definitely no blood stains or cuts on his body.

The reservation was made for just a day, but Nick never formally checked out.

When the staff came into his room later Sunday morning, they found the shower "full of blood" and blood on the bed. The window in the room was covered by bed sheets.

LAPD Robbery-Homicide detectives went to the hotel Monday to gather evidence and also interview employees.

Nick was eventually tracked down and arrested around 20 miles away in Exposition Park, near downtown Los Angeles.

TMZ broke the story ... sources told us Nick was acting very strange at Conan's party ... not interacting with guests, and his attire was out of place ... it was a pretty ritzy event, but Nick was rolling around in a sweatshirt and seemed high on something.

The Reiner's have had problems for years trying to help Nick kick his substance abuse problem ... largely to no avail.