HUNTSVILLE, Texas (AP) _ Three times the frantic voice on the telephone screamed at the 911 operator: ``Help me!''

The dispatcher at the Whitehouse Police Department then heard a loud noise and a man's voice yelling, ``Hang up the phone.'' The woman moaned and whimpered. The line went dead.

Responding to the call, police found the bodies of two women. The man convicted of their shooting deaths 8 1/2 years ago, Thomas Wayne Mason, 48, is set to die Monday night by lethal injection. He would be the 20th Texas inmate executed this year and the first of three set to die this week.

The victims were Mason's former mother-in-law, Marsha Brock, 55, and her 80-year-old mother, Sybil Dennis.

``He was upset with his ex-wife and decided to take it out on her mother and grandmother,'' said Jack Skeen, the district attorney who prosecuted Mason.

Mason, the estranged husband of Brock's daughter, Melinda Mason, was arrested the day after the Oct. 2, 1991, slayings. A shotgun was recovered in a pasture and blood scrapings on the weapon matched the blood of the victims.

Two weeks before the shootings, Mason took his estranged wife hostage and held her for 5 1/2 hours at gunpoint, according to testimony at his trial.

In a recent death row interview, the former drywall installer denied any role in the slayings.

``It didn't happen. I never shot at nobody,'' he said. ``The government is conspiring to murder me.''

On Wednesday, John Burks is set to die for the fatal shooting of a Waco tortilla store owner during a 1989 robbery. Paul Nuncio is scheduled to follow him to the death chamber Thursday for the 1995 rape and strangling of a 61-year-old Plainview woman during a burglary at her home.

The planned executions come at a time of increased debate over capital punishment and criticism of Texas' system of capital punishment. Since reinstating the death penalty in 1976, Texas has executed 218 _ more than any other state and about a third of the country's total.

The Chicago Tribune reported Sunday that dozens of Texas inmates have been executed although their defenses were marred by unreliable evidence, disbarred or suspended defense attorneys and questionable psychiatric testimony.

Gov. George W. Bush, who earlier this month granted his first 30-day reprieve in a death penalty case, defended his state's system of capital punishment.

``I know there are some in the country who don't care for the death penalty, but ... we've adequately answered innocence or guilt,'' Bush said. ``They've had full access to the courts. They've had full access to a fair trial.''