LOS ANGELES — Police believe a 72-year-old insurance claims adjuster arrested earlier this month is the most prolific serial killer in the city's history, having raped and strangled as many as 30 older women over two decades. The break in the cold case came in October when John Floyd Thomas Jr., who had twice been convicted of sexual assault, had a DNA sample taken as part of an effort to build an offender database. Thomas was charged April 2 with murdering 68-year-old Ethel Sokoloff in 1972 and 67-year-old Elizabeth McKeown in 1976, both of whom were sexually assaulted and strangled.