SAN DIEGO - A tow-truck driver discovered a human head in a trash bag off Interstate 5 on Tuesday morning, and two hours later, a Caltrans crew 3 miles away found a hand.

The gruesome discoveries along the I-5 corridor came one day after a man's dismembered body was found floating in a culvert in the Palm City community in South County.

Asked how much of the victim's body had been recovered, San Diego police Lt. Kevin Rooney replied, "We're about halfway there."

Authorities said the freeway service patrol driver was heading south on I-5 about 6:30 a.m. when he pulled off on the right shoulder and stopped just south of

The driver picked up the bag, looked inside and saw the head.

While detectives were investigating that discovery, police said a Caltrans crew preparing for routine landscaping found a hand along the southbound state Route 163 ramp to southbound I-5 about 8:30 a.m.

The hand was on the side of the road, in plain view.

"It wasn't hidden or anything. It was just sitting there," Rooney said.

The hand "appears to be in good condition," Rooney said. "We're hoping we can get a good set of fingerprints so that hopefully, we can learn who this gentleman was. Everything starts from that."

Forensic psychiatrist Richard G. Rappaport, associate professor at the University of California San Diego, said one victim does not create a pattern, but cutting up victims "is the kind of thing a serial murderer does. ... These types are usually under the heading of sexual-sadist murderers."

He cited notorious killer Jeffrey Dahmer, who dismembered at least 11 of his 17 male victims before his 1991 arrest in Milwaukee.

Rappaport consulted on the case of John Wayne Gacy, who killed 33 men and boys around Chicago. Gacy and Dahmer, he said, found some kind of sexual pleasure in their crimes.

"They feel entitled to do it, destined to do it - required, even, in some way," Rappaport said.

He said a serial killer can wait months or years before striking a second time, but that the intervals between victims become shorter after that. A crime-spree killer, on the other hand, slays a number of people in a short time.

Regarding the San Diego case, Rappaport said, "With a single case, without two points of contact, I can't say it's more than an individual event."

The head appears to resemble that of a man whose image was caught by a grocery store surveillance camera. He is believed to be the homicide victim whose dismembered body was found Monday morning in a culvert, Rooney said.

Two workmen on their way to a job discovered the body. They had pulled off the street to look at a map and noticed the remains, partly immersed in the Otay River.

San Diego lifeguards removed the clothed body and searched for evidence.

Rooney wouldn't say how authorities linked the man to the grocery store. A time-and-date stamp on the image indicates it was taken at 4:38 p.m. Friday. The man, possibly in his 60s, was white and balding, with a mustache and glasses.

"From his clothing and the size of his backpack, we theorize that he might have been transient," Rooney said.

Correspondent Pauline Repard contributed to this report.