IT MUST BE a mistake.

That's what Rich Muller's friends were hoping as they walked up Stevens Street toward his home just after dusk yesterday. They'd heard that the 22-year-old college graduate and his mother, Donna, had been shot to death inside their Lawncrest rowhouse.

They just didn't want to believe it.

"He wouldn't hurt a fly. He wouldn't get in fights. It couldn't have been him," said classmate Brian Zukowski. "They don't have much. If it was a robbery, I don't know what they would have got. The hubcaps on his car didn't even match."

Police responded to the home about 3:15 p.m. and found Richard and Donna Muller, 49, both shot in the head. Their bodies were on the second floor, in the hallway and in a bedroom.

Chief Inspector Scott Small said investigators had not determined a motive. Shell casings from a semiautomatic gun were recovered, but not the gun. Police found no signs of forced entry, Small said.

Zukowski, 22, said Rich Muller, who worked in the kitchen at the Holy Redeemer Lafayette retirement community, recently graduated from Manor College in Jenkintown. He loved video games such as "Call of Duty," kept on top of his schoolwork and shot hoops with friends in Fox Chase.

"He would open up his house if you didn't have anywhere to go," Zukowski said, adding that he knew people who had stayed in Muller's basement when they were temporarily homeless.

Friends and neighbors had no theories on why someone would kill the Mullers, execution style. Detectives were interviewing witnesses.

"If you needed a ride, he'd take you, no problem," said John, a friend of Rich Muller's who asked that his last name not be printed. "I have no idea, no clue who would do this. I hope they get the [killers]."

Neighbor John Downey, whose daughter attended Cardinal Dougherty High School with Donna Muller, said there was another shooting on the block a few years ago.

"This whole neighborhood changed pretty quick," Zukowski said.

Zukowski said Muller tended to keep to himself, but would occasionally make an appearance at a party at La Salle University where they had friends. When he did, Zukowski said, "you'd be pleasantly surprised."

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