The McLean man who walked into a Rockville high-tech firm in May, emptied his gun into the company's president and then asked a receptionist to call police will plead guilty next month to first-degree murder, according to an agreement filed yesterday in Montgomery County Circuit Court.

A murder conviction would bring Timothy Chang, 74, a life sentence for the slaying of Tseyang "Jason" Chou, 51. But Maryland law does not provide for a mandatory minimum amount of time that must be served.

Chang's attorneys said yesterday that they will ask Montgomery County Circuit Court Judge S. Michael Pincus, who is scheduled to hear the plea Sept. 27, to suspend some of the life sentence on the grounds that Chou was blackmailing their client and that Chang suffered from brain damage that might have affected his reasoning.

The plea agreement was filed two days before Chang's case was scheduled to go before a Montgomery County grand jury, his lawyers said. By entering a plea agreement before he was indicted, they said, Chang benefited because prosecutors dropped a handgun charge, which would have resulted in a mandatory minimum sentence of five years.

"At sentencing, we'll give a full and complete picture of why this tragedy occurred," said James Salter, one of Chang's attorneys. "Believe me, there was a lot of provocation. There's no question in my mind there was blackmail for a substantial amount of money, for all of his money."

Chang told police that Chou had accused him of having an affair with his wife, Patricia Chou, whom Chang knew through Chinese opera groups, authorities said. Chang told police that Chou had threatened to disgrace him by exposing the alleged affair in the Chinese community if he did not pay him $650,000.

Robert Koven, Patricia Chou's attorney, said his client learned of the plea agreement yesterday afternoon. "She's overwhelmed now, almost as much as she was when she heard about the shooting," Koven said.

Montgomery State's Attorney Douglas Gansler said yesterday that Chang's motive "is still kind of unclear" and that prosecutors "haven't established anything one way or another" about whether Jason Chou had blackmailed Chang.

"Even if he [Chang] were being blackmailed, the appropriate remedy is not to go out and buy a gun and murder someone in cold blood," Gansler said.

Gansler said Chang bought a .38-caliber revolver a few days before the slaying and learned to use it by target-shooting at a range.

"The only issue," Gansler said, "is what do you do with a 74-year-old man with no prior record who commits the highest degree of a crime?"

Chang and Patricia Chou became friends through the Chinese opera, family friends said. Chang was well-known in local Chinese opera circles as a competent player of an ancient two-string violin, and Patricia Chou sang in a rival opera company. Jason Chou was president of Imacom Inc., a small medical imaging firm.

Salter said that Chang considered Patricia Chou a "goddaughter," similar to a surrogate daughter in the Chinese culture, and that they were not romantically involved. Friends from the Chinese opera have said they believe Chang may have felt he was rescuing Patricia Chou from a bad marriage.

Chang's attorneys initially had pursued a possible insanity defense, but doctors did not find any mental illness serious enough to meet the legal threshold for insanity, said Judith Catterton, one of Chang's attorneys.

However, Catterton said, doctors found that Chang has "very significant brain damage" to his frontal lobes, which could have affected his decision-making and memory. She said the brain damage may have stemmed from a traffic accident in which Chang was knocked unconscious or from hardening of the arteries due to aging or high blood pressure.

"We're interested in how it affected his functioning at that time," Catterton said.