For two years, authorities say, a retired Army helicopter pilot picked up prostitutes here, shot them in the head and robbed them, then covered their faces with plastic bags and dumped the bodies along dirt roads and vacant fields.

Robert Lee Yates Jr., 47, sat stone-still in a Spokane courtroom Friday as prosecutors charged him with eight counts of first-degree murder and one count of attempted murder. Four more murder charges could be filed soon, said authorities who are investigating Yates in connection with the killings of 18 women in Washington state since 1990.

The nine crimes Yates has been charged with cover a two-year period after he retired in Spokane. Because experts doubt that a serial killer would suddenly begin such brazen and methodical killings at age 44, the FBI is helping investigators in 11 states and two countries compare Yates's military travels with the unsolved slayings of dozens of women around the world over the last two decades.

Detectives investigating the homicides and disappearances of 31 prostitutes in nearby Vancouver have begun looking at Yates. And authorities in Germany, where Yates served two tours of Army duty from 1980 to 1984 and from 1988 to 1991, see him as a potential suspect in as many as 26 unsolved homicides.

"In general . . . people don't wake up one day and decide, 'I think I'm going to be a serial killer,' " said FBI supervisory special agent Mark Safarik, a veteran profiler in Quantico, Va. "This is usually behavior that has developed from when they were very young."

In court documents filed this week, Spokane County investigators allege that after moving to Spokane in 1996, Yates picked up prostitutes, shot them in the head, robbed them and put plastic bags on their heads, possibly to control the bleeding.

If Spokane prosecutors have a key witness, it may be Christine Smith, a 32-year-old prostitute who came forward recently after seeing a picture of Yates in a newspaper. She offered a chilling tale of escape from a man she says was Yates.

At 1 a.m. on Aug. 1, 1998, Smith was working as a prostitute near a beauty supply store when she was picked up by a man in a black 1970s van. She said she asked the man if he was "the psycho killer."

In a police report filed at the time of the incident, Smith remembered the man saying "he was not the killer because he had five kids and would not do that" and that "he was a helicopter pilot with the National Guard."

After retiring from the Army, Yates was a National Guard pilot. He is married and has five children. Among a dozen or so vehicles that investigators have searched is a black 1979 Ford van that Yates owned between 1997 and 1999, which seems to match a detailed description that Smith gave in 1998.

According to Smith, the man gave her $40 to perform oral sex but did not become aroused before Smith felt a blow to the back of her head. As she struggled to remain conscious, the man asked for his money back. Smith edged toward the front of the van and escaped.

Smith received three stitches and assumed that she simply had been hit in the head during a robbery attempt. But this March, an X-ray after a traffic accident revealed that she apparently had been grazed by a bullet and still had fragments of it in her head, according to court documents.

After the fragments are removed from Smith's head, detectives plan compare them with a .25-cal. bullet and shell casing they found in the van. At least seven of the other victims were killed with the same brand of .25-cal. bullet.

"It was amazing to have this witness come forward," said Spokane County Sheriff Mark Sterk. "She gives us some insight into the pattern here and what some of the victims may have gone through."

Yates is being held on a $1.5 million cash bond in the Spokane County jail, where he has been visited by his wife and at least some of his children. Spokane County public defender Richard Fasy said Friday that Yates will plead not guilty when he is arraigned May 31.

Prosecutor Steve Tucker said he will decide in the next six weeks whether to seek the death penalty on the charges of first-degree murder with aggravating circumstances.

"It looks like a tight case," Tucker said.

Police have matched Yates's DNA to semen found on at least seven bodies. Experts say the chance that the semen came from someone other than Yates is 1 in 221 million.

Investigators also say that Yates's fingerprint was on one of the plastic bags placed over a victim's head and that they have home video of what appears to be Yates's van taken near the strip where prostitutes work on the night Christine Smith was attacked.

Detectives collected lawn clippings and other vegetation with the bodies that an expert says came from Yates's yard, and a white Corvette that Yates once owned, in which detectives found blood from a slain 16-year-old, Jennifer Joseph, and a button from her shirt.

But for investigators of other unsolved killings who have requested information about Yates, the most valuable evidence may be the record of his 18 years in the Army and two years in the National Guard.

Investigators say Yates was on weekend National Guard duty in Tacoma, on the other side of Washington state, in December 1997 and October 1998, when two women were killed there.

FBI agent Norm Brown said he is collecting everything from bank records to travel vouchers to help Spokane detectives create a time line that other agencies can use. Because of his military duty, Yates traveled "more than your normal citizen," said Brown.

The Pentagon this week confirmed that Yates served in Army operations in Somalia in late 1992 and early 1993 and in Haiti in 1994. Among the other places he was stationed was Fort Drum in New York and Fort Rucker in Alabama. Yates is being investigated for unsolved slayings near both bases.

Among the other states where investigators have requested information on Yates are Florida, Illinois, Massachusetts, Idaho, California, North Carolina, Wyoming, Montana and Colorado. In all, 30 police agencies have expressed interest.