EL CAJON (CNS) - A Chula Vista man already facing charges that he murdered his wife and mother-in-law was charged Wednesday with murdering a third person.

At the request of the defendant's attorney, information about the latest charge was sealed by the court.

Michael Eugene Richardson, 57, was charged last year with the deaths of his 39-year-old wife Thao and 72-year-old mother-in- law, Than Lyi.

The defendant was scheduled for a preliminary hearing Wednesday, but prosecutor Kurt Mechals filed an amended complaint charging Richardson with a third murder.

El Cajon Judge Peter Deddeh granted a request by Deputy Public Defender Richard Gates that the amended complaint be sealed until a preliminary hearing, which was rescheduled for May 24. Richardson is expected to be arraigned on the latest charge at that time, as well.

District Attorney Bonnie Dumanis has yet to decide whether to seek Richardson's execution or life in prison without the possibility of parole. The charges include an allegation of multiple murders, making him eligible for the death penalty.

Richardson also is charged with carrying on a sexual relationship with his teenage niece. He was charged last summer with three counts of statutory rape, eight counts of oral copulation with a minor, and one each of rape with a foreign object and attempting to dissuade a witness stemming from the alleged abuse of his 17-year-old niece.

At the time, authorities said Richardson was under investigation for the deaths of his wife and mother-in-law, who were found dead June 29 in a wrecked car off state Route 67 north of Lakeside.

Investigators determined their injuries were not caused by a crash and called in homicide detectives.

At Richardson's arraignment in July, Deputy District Attorney Chantal de Mauregne told a judge that Richardson escaped from a North Carolina prison in 1982 - where he served time for robbery - and remained on the lam for 18 years.

The prosecutor said Richardson had assets of $10 million and had recently shuffled $500,000 between various bank accounts.

The defendant's niece and her mother moved from Vietnam to Richardson's home in December, and the defendant quickly initiated a relationship with the girl that started with hugs and kisses and progressed to sexual activity in the first three months of 2010, the prosecutor alleged.

De Mauregne said the defendant's wife discovered what was happening when she read text messages between the two.

Richardson was arrested in March, but released without charges when the girl initially refused to cooperate.

The teen told authorities "she was in love with the defendant, who said he would take care of her," the prosecutor said. "According to the victim, the defendant promised her he would leave his wife, be with the victim, and basically run off with her."