DNA leads detectives to John Thomas Jr., 72. He is held in two slayings, but police suspect he may have killed up to 30 elderly Westside and Claremont women a decade apart.

The first wave of slayings haunted Los Angeles in the mid-1970s. The killer slipped mostly unseen through the night, preying on older women who lived alone. He raped them and squeezed their necks until they passed out or died. On the 17 who were killed, he placed pillows or blankets over their faces.

The second wave hit a decade later in Claremont -- five older women raped and strangled, faces again covered.

Even with at least 20 survivors, police never connected the two homicide-and-rape rampages nor solved either of them. The victims gave conflicting descriptions of the rapist, police in different jurisdictions didn't communicate, and DNA technology had not come into use.

Now authorities say they have linked John Floyd Thomas Jr., a 72-year-old state insurance claims adjuster who twice has been convicted of sexual assault, to five of the slayings. Detectives also describe him as a suspect in up to 25 more based on the circumstances of those crimes.

"When all is said and done, Mr. Thomas stands to be Los Angeles' most prolific serial killer," said LAPD Robbery-Homicide Cold Case Det. Richard Bengston.

Thomas was arrested at his apartment in South Los Angeles last month and charged April 2 with murder in connection with the deaths of Ethel Sokoloff, 68, in the Mid-Wilshire area in 1972, and Elizabeth McKeown, 67, in Westchester in 1976.

He said Thomas' DNA matched evidence found at five murder scenes, spanning both crime waves -- the two homicides he has been charged with, one in Lennox in 1975, one in Inglewood in 1976 and one in Claremont in 1986.

Authorities are analyzing evidence in 25 other killings they suspect might be linked to Thomas.

Thomas had been working as an adjuster handling workers' compensation claims since 1989 -- the year the killings stopped. He resigned after his arrest March 31.

Jennifer Vargen, a spokeswoman for the State Compensation Insurance Fund, would not comment on whether the employer was aware of Thomas' criminal record, saying it was a personnel matter.

Co-workers at his office in Glendale described Thomas as quiet but friendly. They said his job mostly involved paperwork.

His steady employment masked a troubled past.

Thomas was born in Los Angeles. His mother died when he was 12 and he was raised by his aunt and a godmother. Thomas attended public schools, including Manual Arts Senior High School.

He briefly joined the U.S. Air Force in 1956. At Nellis Air Force Base in Nevada, a superior described Thomas as often late and slovenly. He received a dishonorable discharge, according to his military records, and was arrested for burglary and attempted rape in Los Angeles. He was convicted and sentenced to six years in state prison in 1957. Two parole violations sent him back behind bars until 1966.

The first wave of rapes began a few years later. The so-called "Westside Rapist" attacked white seniors, in neighborhoods from Hollywood in the north to Inglewood in the south. The crimes led to the formation of a special police task force in the mid-1970s.

The LAPD questioned several suspects in those slayings. Thomas was not among them. During this period he was employed as a social worker, hospital employee and personal electronics salesman.

The "Westside Rapist" became one of the more notorious criminals of the era. Victims ranged in age from the 50s to the 90s. Bella Stumbo, the late Times feature writer, wrote in December 1975 that the "serenity" of the neighborhoods where the victims lived "had been so grotesquely invaded by that elusive maniac the police loosely refer to as the 'Westside rapist,' now accused of sexually assaulting at last 33 old women and murdering perhaps 10 of them." She said residents lived in "small colonies of terror."

The attacks appeared to stop in 1978. That year, a witness took down Thomas' license plate after he raped a woman in Pasadena. He was convicted and sent to state prison.

When he was released in 1983, he moved to Chino. And a killer began stalking older woman -- this time in the Inland Valley area.

Over the next six years, Los Angeles County sheriff's detectives would investigate five slayings of elderly women in Claremont, Sgt. Richard Longshore said.

During that period, Thomas worked in neighboring Pomona as a peer counselor at a hospital.

Detectives now believe the last in this cycle of killings occurred in 1989. They are not sure why the perpetrator stopped. That year, Thomas took a job in the state workers' compensation agency in Glendale.

Over the next two decades, the Westside Rapist faded from public memory, and authorities made little headway in the Claremont killings.

In November 2001, the LAPD created the Cold Case Homicide Unit to reopen about 9,000 unsolved slayings going back to 1960, using emerging state and federal DNA databases.

In September 2004, the department's crime lab matched male DNA taken from both the McKeown and Sokoloff crime scenes, police said. But they couldn't match the DNA to a suspect. Over the next five years, detectives developed 14 suspects, but their DNA ruled each of them out as the attacker.

The break came last October, when two officers collected DNA from Thomas as part of an ongoing process to swab registered sex offenders. On March 27, the California Department of Justice DNA Laboratory notified detectives that his DNA matched the evidence from the Sokoloff slaying.

On March 31, they were told that his DNA matched the four other slayings. He was arrested later that day. Thomas is being held at L.A. County Jail and could not be reached for comment.

Police said that connecting the dots in such cases was much harder before DNA and computer databases.

"It was harder to make connections," said LAPD Deputy Chief Charlie Beck. "The difference in investigative techniques, communication and the science is huge."

It has become standard practice for investigators to collect evidence such as hair, fingernail scrapings, and bodily fluids from murder and rape victims.

DNA databases have contributed to a number of arrests and convictions since the beginning of the decade. Several years ago LAPD detectives arrested Chester Dewayne Turner, who was responsible for 10 rape-strangulations along the Figueroa Street corridor in South L.A. and in downtown.

He was convicted of the murders and sentenced to death in May 2007.

But the technology is limited unless a perpetrator's genetic code lands in the database.

The LAPD is still investigating at least a dozen murders, over a span of two decades, connected to an unidentified serial killer dubbed the "Grim Sleeper."