The neighbors on Sydney Street in East Mount Airy had wondered about the unfamiliar car with dark windows that sat on the block all weekend. One said the engine had been idling for two days. Another said he had passed by and glimpsed two figures in the front seat.

On Sunday afternoon, someone called the police, who opened the car to a grisly scene: A man, 24, and a woman, 21, both shot once in the back of the head, likely by someone in the car with them, Homicide Capt. James Clark said.

Clark said it appeared the victims knew their assailant. But the case, he said, was "very much a whodunit."

"There's no clear motive," he said. He did not release the victims' names, and said they were from the Philadelphia area but not necessarily from the neighborhood, the 1000 block of Sydney Street.

Police on Sunday night were trying to learn how long the two had been in the car. Its tinted windows made it difficult to see inside, police said.

One woman who lived on the block, who declined to give her full name, said another neighbor had peered into the car Sunday afternoon, saw the bodies inside, and called police.

Longtime residents, who gathered on the street corner as crime scene investigators prepared to tow the car to the morgue Sunday night, said the block was a quiet one. "It's tragic," said Sam Witherspoon, who lives down the block from where the victims were found. "I've been living here 20 years, and this is the first time ever something like this has happened."