A McKeesport woman was shot to death early yesterday by two robbers who were seeking valuables and marijuana, Allegheny County homicide detectives said.

The accused killer described how he shot Tammi Heckman when she tried to flee during the robbery at her home in the 200 block of Perry Street, according to an affidavit filed by police. When questioned by investigators, Michael Evans, one of the two suspects, admitted that he fired three shots at Ms. Heckman, the affidavit states.

The first bullet missed. She was struck by the second round, and the third shot "put her down," Mr. Evans is quoted as telling investigators.

Ms. Heckman, 40, was pronounced dead on the back porch of her home, police said. The Allegheny County medical examiner's office said she died of multiple wounds to the trunk.

Mr. Evans, 24, of Homewood, and the other suspect, Calvin Loving, 22, of Wilkinsburg, were arraigned last night on charges of homicide, robbery and conspiracy. Mr. Evans also was charged with a weapons violation because, as a convicted felon, he was prohibited from possessing a firearm. He pleaded guilty in August 2007 to committing a burglary.

Police were summoned at 1:28 a.m. yesterday and were met by several witnesses who directed them to Ms. Heckman's body. Witnesses also reported the suspects were seen carrying a bag when they left her house.

One witness told police he followed the suspects as they hurried away. The witness still was following the men when McKeesport police spotted them walking along Erie Street.

Mr. Loving told investigators that he and Mr. Evans had planned the robbery for about two weeks. They believed a man who also lived at the house kept pounds of marijuana there, police said. That man's relationship to Ms. Heckman was not clear yesterday.

The two encountered Ms. Heckman outside the house and ordered her inside, where Mr. Evans pulled his pistol and forced her to lie face down on the kitchen floor. Mr. Loving already had begun searching the residence for money -- as much as $50,000 -- drugs and valuables, according to the affidavit.

Mr. Loving told police he'd just walked down the stairs from the second floor when he heard three shots. Mr. Evans told police that he had kept Ms. Heckman under guard, but he shot her when she jumped up and tried to run, according to the affidavit.

Police said Mr. Loving also turned over valuables that he said he took from Ms. Heckman's home. The two men are being held without bail in the county jail, pending a preliminary hearing scheduled for Friday.