GOLETA, Calif. - A former postal worker shot five people to death at a huge mail-processing center and then killed herself in what was believed to be the nation's deadliest workplace shooting ever carried out by a woman.

The attack Monday night was also the biggest bloodbath at a U.S. postal installation since a massacre 20 years ago helped give rise to the term "going postal."

Investigators would not release the killer's name or discuss a motive or the circumstances under which she left the Postal Service two years ago.

The rampage the nation's first deadly postal shooting in nearly eight years sent employees running from the sprawling Southern California complex and prompted authorities to warn nearby residents to stay indoors as they searched for the killer.

Authorities found two people dead outside the plant, blocks from the University of California, Santa Barbara. Another body was just outside the door, and a fourth was just inside. Two more bodies including that of the killer were farther inside.

A sixth person was in critical condition with a gunshot wound to the head.

All of the woman's victims were believed to be employees at the postal center.

The woman reloaded her 9 mm handgun at least once during the rampage, Santa Barbara County Sheriff Jim Anderson said. The woman had not worked at the plant for more than two years but still managed to get inside the fenced and guarded Santa Barbara Processing and Distribution Center, Anderson said. How she got in was under investigation.

James Alan Fox, a criminologist at Northeastern University in Boston and an expert on homicides, said the death toll might be the highest ever for any workplace shooting carried out by a woman.

"Men, more than women, tend to view their self-worth by what they do" at work, Fox said. Men also appear more prone to use violence in seeking revenge while "women tend to view murder as a last resort," he said.

Postal employee Charles Kronick told KEYT-TV in Santa Barbara that he was inside the building when he heard shots. "I heard something that sounded like a pop, and then I heard a couple seconds later, another pop, pop, pop," Kronick said.

Kronick said his boss came running over and told him to get out of the building, and "we all hightailed it out real quick."

Some 50 to 60 employees were seen running from the plant, and many fled to a fire station across the street.

Postmaster General John E. Potter said counselors would be available to the families and employees. "Our heartfelt prayers and condolences go out to the families of the victims and to our employees who have suffered through this tragic incident," he said in a statement issued in Washington.

It was the deadliest shooting at any U.S. workplace since 2003, when 48-year-old Doug Williams gunned down 14 co-workers, killing six, and then committed suicide at a Lockheed Martin aircraft parts plant in Meridian, Miss.

It also was the bloodiest rampage at a U.S. postal installation since 1986, when a mailman killed 14 people in Edmond, Okla., and then took his own life. Postal installations were hit with a string of deadly shootings in the mid-1980s and early '90s.

The nation's last postal shooting was in 1998, when a 27-year-old letter carrier fatally shot a post office clerk in Dallas after they argued in a break room. The gunman was found not guilty of murder by reason of insanity.

About 300 people are employed at the Goleta plant, situated about a mile from the ocean, with the mountains as a backdrop. Goleta (pronounced goh-LEE-tuh) is about 90 miles northwest of Los Angeles.

Employees were told to report Tuesday to another processing center in nearby Oxnard.