L.A.'s "Southside Slayer" Michael Hughes convicted of three more murders

(CBS/AP) LOS ANGELES, Ca. - The man previously convicted of four "Southside Slayer" serial killings has been found guilty of strangling three more victims over a 10-year period.

On Thursday, former security guard Michael Hughes, 55, was convicted of three counts of first-degree murder with special circumstances for strangling two prostitutes and a 15-year-old girl during a series of killings that took place in the Los Angeles area during the 1980s and 1990s.

Hughes was already serving a life sentence for killing four women in 1992 and 1993 when he was found guilty for these additional murders on Thursday.

Prosecutors will now seek the death penalty when the sentencing trial begins on Monday.

In 1986, 15-year-old Yvonne Coleman's body was found and jurors Thursday decided Hughes had killed her and that the crime was committed during a rape and sodomy.

He was also accused and found guilty of murdering two prostitutes, Verna Williams and Deborah Jackson, in 1986 and 1993. Police said DNA evidence links Hughes to the killings.

Along with murder, jurors found Hughes guilty of special circumstances because of his prior first-degree murder convictions.

Norman Kallen, his defense attorney, said there were no eyewitnesses to the murders and that Hughes never confessed.

Hughes was investigated by a task force looking into three separate serial killers accused of stalking women during a crack cocaine epidemic that forced women onto the streets as prostitutes to feed their addictions. As many as 90 women were killed in inner-city Los Angeles during that time.