A Silver Spring man who shot his estranged girlfriend to death Monday killed himself with a shotgun blast at his home early yesterday after telling Montgomery County police during a lengthy standoff that he had planned a murder-suicide and would not be talked out of it, authorities said.

Howard Smith, 33, fled to the brick, split-level home off Venice Drive after shooting Dorothy Gaynaille Lewis, 25, outside a Rockville delicatessen Monday afternoon, Police Chief Bernard D. Crooke Jr. said. Holed up in his bedroom, he spoke with a police negotiator by telephone late into the evening while heavily armed officers surrounded the house and others directed traffic away from the tidy, middle-class neighborhood off New Hampshire Avenue.

Described by neighbors as a high school dropout and an odd-jobs laborer, Smith repeatedly urged police to storm the house, said Sgt. Joseph Hock, who spent hours trying to coax him into surrendering. Hock said Smith told police he had planned from the beginning to die after killing Lewis, whom he accused of infidelity.

"He insisted he was going to die," Crooke said. "He was bound and determined. He had thought it all out. He was going to kill himself."

Crooke said Smith put a shotgun to his head sometime after 12:15 a.m. yesterday, after police had decided further negotiations would be fruitless and filled the house with tear gas, hoping to force him out.

But he never emerged. About two hours later, the chief said, officers moved inside and found Smith dead in an upstairs bedroom.

By then, Detective John Cady said, nearly 12 hours had passed since Smith went looking for Lewis with a .22-caliber rifle, spotted her outside the Hungerford Beer, Wine & Deli, and shot her several times -- once at close range in the back of her head as she lay on the pavement.

Cady described Lewis as an occasional "street person" with a young son in foster care. When not roaming Rockville's streets, she stayed with acquaintances, Cady said. He said he was unsure when Lewis and Smith met. But at times, he said, the couple lived together in the Venice Drive house, which is owned by Smith's father, Walter.

"When they would squabble, she'd go out and live on the street," the detective said. "And he would go out and find her, and they'd make up, and she'd come back and live with him for a while."

He said several witnesses at the deli shooting noted the license plate number of Smith's brown Chevrolet Chevette. Police traced it to the Venice Drive address, he said. The first officer to arrive spotted Smith rushing into the house and summoned help, including the department's Emergency Response Team -- negotiators and heavily armed officers who specialize in barricade situations.

Hock, one of the department's trained negotiators, said he arrived on Venice Drive minutes after the standoff began and immediately reached Smith by telephone.

In an interview yesterday, Hock gave this account of their initial conversation:

"Is this Howard?" Hock asked.

"Yeah," came the reply.

"My name is Joe Hock, with the Montgomery County Police Department, and we want you to come out . . . . "

Smith sounded agitated. "Is she dead yet?" he demanded. "Is she dead yet? I tried to call the hospital."

As Hock tried to change the subject, Smith persisted. "I did it. I planned it. I'm going to kill myself," he told the officer. And when Hock again asked him to surrender, Smith said, "No, I'm not coming out. You're going to have to come and get me."

As officers surrounded the house with shotguns, high-powered rifles and submachine guns, Hock and Smith carried on a series of discussions as hours passed. At other times, Hock said, Smith called acquaintances and neighbors, or spoke with those who called him.

Smith cried at times, Hock said, and at other moments, he sounded angry. But throughout the standoff, he was defiant about giving up.

"We tried leaving him alone, we tried talking to him a lot, we tried isolating him -- we tried everything," Crooke said. "We gave him many hours and every opportunity for an honorable surrender."

About 9 p.m., he said, police restricted Smith's telephone calls, allowing him to communicate only with the negotiators' command bus. The police cut off electricity about 11:30 p.m., he said, after Smith became agitated by television news reports of the standoff.

Finally, he said, they chose tear gas. It was Crooke's decision.

"We were getting nowhere," the chief said, "and we had a murder suspect who needed apprehending."