The FBI and two investigators from the National Center for Missing and Exploited Children joined the search for a 19-year-old college student who mysteriously vanished from a Reno, Nev., home early Sunday morning.

Brianna Denison, a student at Santa Barbara City College, was staying with friends near the University of Nevada, Reno. She last was seen at about 4 a.m. Sunday, when she returned from a party and went to sleep on the couch wearing sweatpants and a T-shirt.

A spot of what appeared to be blood was found on her pillow. Police have sent it to the lab for testing. The couch where Denison was sleeping was in front of an unlocked glass door. Investigators believe she may have been abducted.

"We have not come up with any other clues, any of the items or anything that would lead us to Brianna at this time," police Lt. Ron Holladay said. "All the indicators are that Brianna has been abducted. It is our sincere hope that she'll come walking in the door any minute, but the longer it goes, the less the chances are of that occurring."

Police interviewed the man who drove a friend of Denison's home after their night of partying and said he is not considered a suspect.

Sgt. Gerald Rhodes, head of the regional Sex Offender Notification Unit, told the Reno Gazette-Journal his detectives were interviewing registered sex offenders who live within a mile of the house from where Denison went missing, as is standard practice.

Click here to view the full Reno Gazette-Journal report.

The newspaper reported police received many tips, but none has panned out.

Denison is a psychology student at Santa Barbara City College in Santa Barbara, Calif., and was back in her hometown of Reno for winter break.

In the hours before her disappearance, Denison had been party-hopping at venues associated with a weekend snowboarding event, including attending a rap concert at the Sands Hotel and Casino.

Denison's family is offering a reward for information leading to her safe return.

Her aunt, Lauren Denison told FOX News that her niece's disappearance was very unusual.

"If she could contact us, she would," said Lauren Denison, adding that Tuesday was a critical day in the search. "If anyone is watching, we are desperately wanting her back, no matter what. If anyone knows anything, please call the police department.'

Denison's friend K.T. Hunter told FOX News that it didn't initially occur to her and her housemates that something might have happened to Denison when they saw she wasn't on the couch Sunday morning.

"It didn't even cross our mind," said Hunter, who thought Denison went to sleep in her roommate's vacant room. "When we knocked on my roommate's door and she wasn't there, we knew something was wrong."

Hunter said there was no sign of a struggle, but they found a blanket Denison was using in the kitchen.

She said her friend's disappearance has been "a nightmare."

"At first, I didn't even believe it was happening," a tearful Hunter told FOX News. "As time goes on, I can't imagine another minute without knowing where Brianna is. Everyone just loves her so much."

Investigators say she may have been wearing light blue or pink sweatpants, a T-shirt and no socks or shoes when she disappeared. Her purse and cell phone were found in the house, but a brown stuffed bear that she was using as an extra pillow is also missing, police said.

Investigators were looking into the possibility that Denison's disappearance is related to attacks against two female students at the University of Nevada, Reno, late last year.

Both women were grabbed from behind and sexually assaulted in the early morning hours in November and December. No arrests have been made.

The Associated Press contributed to this report.