LOS ANGELES , California ( CNN ) -- A man who Los Angeles police believe raped and murdered dozens of women decades ago was arrested by cold case investigators this month after a computer matched his DNA to evidence from two killings in the 1970s . John Floyd Thomas Jr. may have begun his killings as far back as 1955 . John Floyd Thomas Jr. , 72 , may have begun his killings as far back as 1955 and he could be one of the worst serial killers in United States history , according to Los Angeles Police Chief William Bratton . `` We have yet to reach the depths of what he has done , '' Bratton said Thursday . Until his April 2 arrest , Thomas was a Los Angeles insurance adjuster . Police now call him the `` Southland Strangler '' -- named for the geographical section of Los Angeles County where they suspect he killed at least 30 women and raped many more . Thomas , who sits in a Los Angeles jail , has been charged with two killings -- in 1972 and 1976 -- but prosecutors will likely add more cases when he faces arraignment on May 20 , Bratton said . While Thomas was arrested `` a number of times between 1955 and 1978 '' for sex crimes and burglaries , detectives did not have the technology to identify him as a suspect when the region was terrorized by a series of killings then blamed on the `` Westside Rapist , '' Bratton said . Watch why the suspect may be L.A. 's most prolific serial killer » . Officials , using new computer databases and software , are now `` looking to see what the patterns were , '' said Los Angeles Police Deputy Chief Charlie Beck . `` A lot of work has yet to be done , '' Beck said . Bob Kistner had just begun his law enforcement career in 1976 when his great aunt , 80-year-old Maybelle Hudson , was beaten , raped and strangled in the garage of her Inglewood , California , home . He had just retired as a sergeant with the Long Beach , California , Police Department when he got the call recently that investigators linked Thomas to her murder . `` I waited my entire career for that phone call , '' Kistner said . It was a routine call to Thomas from an LAPD officer last fall that led to the break in the case . Thomas , a registered sex offender , is required by California law to provide a DNA sample for inclusion in the state 's database . Because of a backlog of cases , Thomas was not asked until October to report to a patrol station to have the inside of his cheek swabbed . `` He was very cooperative , '' the patrolman who took the sample said . The California Department of Justice called LAPD cold case detectives on March 27 to tell them the DNA came up as a match to rape kit evidence collected from Ethel Sokoloff , who was 68 in 1972 when she was found beaten and strangled in her Los Angeles home . Those detectives had sent the biological evidence from the Sokoloff case to a state lab in 2002 as part of their review of about 6,000 unsolved murders in Los Angeles that happened between 1960 and 1996 . DNA analysis in 2004 concluded that Sokoloff 's killer also beat , raped and killed Elizabeth McKeown , 67 , in 1976 , Beck said . The murders of three other older women -- including Maybelle Hudson -- were also linked by DNA to a common killer , he said . `` Because of Thomas 's criminal background , the close proximity of his homes to murder locations , similar victim descriptions [ white elderly females ] and other evidence that suggests the type of modus operandi used by the suspect , detectives strongly believe Thomas is very likely the suspect in 'The Westside Rapist ' cases , '' a police statement said . Thomas is single , although he has been married five times , police said . While he served about 12 years in prison between 1955 and the late 1970s for his previous convictions , he has no record since his last arrest in 1978 , police said . Deputy Chief Beck said the growing use of DNA databases and computers to match them to crime evidence will likely lead to more cold case killers being identified .
Police say John Thomas Jr. may have begun killing as far back as 1955 . Thomas arrested April 2 and is charged with killing two women . 72-year-old could be linked to dozens of other killings and assaults .