Her mother was on the hallway floor, police said. Richard was in his bedroom, also on the floor. Both had been shot in the back of the head.
Kaitlyn was not in the home at the time, a police source said.
Neighbors heard screams first, then sirens.
John Downey, who lives two doors away, was sitting down to watch the football game when he heard the sirens.
"I don't know what to say," he said, standing on his porch, as crime scene officers went in and out of the Muller's front door. "It's just a shame. They are nice people. They don't deserve this."
Investigators were still at the scene late Sunday.
Police had yet to develop suspects or a motive, said Chief Inspector Scott Small. But lacking a weapon, they quickly ruled out the possibility of a murder/suicide, Small said.
There were no signs of forced entry, he said. And it did not appear that the house had been ransacked or robbed, the police source said.
Police recovered spent shell casings from a semiautomatic handgun. Neighbors told investigators that they had heard a disturbance outside the Muller house overnight Saturday. Downey said his wife had heard "banging and noise." Another neighbor told police that at one point, he heard two gunshots.
Police have not determined the exact time the Mullers were killed, the police source said, meaning the mother and son may have been dead for hours before they were found.
On Sunday night, William Fullerton, 22, who described himself as Richard's best friend, heard the news and ran to the house, hoping it wasn't true. He ducked under the crime scene tape and tried to get into the Muller home, only to be held back by a police officer as he began to cry.
Friends since age 12, he and Richard had gone to St. William elementary together and then Cardinal Dougherty, where Muller played football, Fullerton said.
In December, Richard Muller had earned an IT degree from Manor College in Jenkintown, he said. For a few years, he had worked as a cook at the Holy Redeemer Lafayette nursing home, Fullerton said.
Besides getting busted once by police for underage drinking, Richard Muller was not one to get into trouble, Fullerton said. He did not mess with drugs like other friends who grew up in the neighborhood, which has grown rougher in recent years, Fullerton said.
"He was so family-oriented," Fullerton said, remembering how Richard, whom friends called Roon, would often leave friends to spend time with grandparents who lived a few blocks away.
Brian Zukowski, 22, who attended Manor College with Richard, stood behind the crime scene tape, having come after hearing about his friend's killing on TV.
When he wasn't working or at school, Zukowski said, Richard spent most of his time at home - doing school work, playing video games such as Call of Duty - or playing basketball with friends at a court in Fox Chase. They had all used to shoot hoops together at the Lawncrest Recreation Center just around the corner, Zukowski said, but stopped a few years ago when there was a double shooting at the court.
"The neighborhood had changed pretty quickly," he said. "He didn't hang around here much . . . He didn't come out every weekend. You'd be pleasantly surprised if you saw him at a party."
On her Facebook page, alongside pictures of her granddaughter, Donna Muller liked to post photos of serene landscapes and snowcapped cities she found on travel magazine websites.
And in December, she posted photos of Disney World castles from her trip with Heather, Heather's boyfriend, and Kaitlyn.
According to her page, Donna Muller was a senior editor at Reed Technology, a patent and trademark office in Horsham.
Like her son, she had attended Cardinal Dougherty, said Downey, the neighbor.
As he talked, flashes of crime scene investigators' cameras could be seen behind the silhouetted windows of the Mullers' upstairs windows.
Anyone with information is asked to call the Homicide Unit at 215-686-3334.