Nick Reiner’s hotel room full of blood after Rob and Michele’s murder

Nick Reinerwho has been arrested for the murder of Rob And Michelle Reinerchecked into a Santa Monica hotel early Sunday morning and left a trail of blood… sources with direct knowledge tell TMZ.

Nick checked into The Pierside Santa Monica around 4 a.m. Sunday using his credit card. His check-in came hours after he got into a heated argument with his father Conan O’BrienChristmas.

Eyewitnesses who saw Nick check in say he seemed “knocked out” but there were no visible signs he had been in a violent confrontation – we’re told for sure there are no bloodstains or cuts on his body.

The reservation was only made for one day, but Nick never formally checked out.

When staff entered his room later Sunday morning, they found the shower “full of blood” and blood on the bed. The window in the room was covered with sheets.

LAPD robbery and homicide detectives went to the hotel Monday to collect evidence and also interview employees.

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

TMZ broke the story…sources told us it was Nick act very strange at Conan’s party… no interaction with guests, and his clothes were out of place… it was a pretty fancy event, but Nick was rolling around in a sweatshirt and appeared to be high on something.

The Reiner’s have been having trouble helping Nick overcome his drug problem for years… largely to no avail.