It has been a question with no answer for their family, friends and police investigators. February 19th, 2015 marks 10 years since they vanished.
It's also the day family and the FBI again asked the public for help.
"10 years is too long not to have justice for two people who did nothing wrong, who didn't deserve this," said Petrone's mother, Marge, said on Thursday.
The FBI says their disappearance was no accident, and not a crime of opportunity. They say Petrone and Imbo were the victims of a professional hit job at the hands of more than one person.
"This didn't just happen. We feel this was an orchestrated act. A 3,000 pound truck and two people do not simply go missing," said Christian Zajac of the FBI.
The FBI has questioned Imbo's estranged husband, Joe Imbo, many times. Just days before he vanished, Petrone told his father he had angry words with Joe Imbo over the phone.
"He said "I got, like, 25 or 30 phone calls from this kid.' I said, 'What did he want?'" Richard Sr. said to Action News last week. "'He kept telling me 'stay away from her, leave her alone, he's her husband.''"
Joseph Imbo has never been named as a possible suspect, but he hasn't been ruled out either.
"We are going back, we are looking at all the leads were covered in the past," said Zajac.
Joseph Imbo spoke to Action News reporter David Henry on Thursday. He said the FBI has "gotta do what it has to do" and said he told agents he's "glad" they are investigating the case and hopes this is finally the year they find out what happened.
Petrone and Imbo vanished after leaving a bar in the 400 block of South Street in Philadelphia - then known as Abilene's - at 11:45 p.m. on Saturday, February 19th, 2005.
They were headed to Imbo's home in Mount Laurel, New Jersey. No one knows if they ever made it there.
No evidence of their fate has emerged in the years since.
Nor has there been any sign of Pettrone's vehicle, a 2001 black Dodge Dakota pickup truck with Pennsylvania license plate YFH 2319.
Petrone's parents, Richard Sr. and Marge, reflected on the painful anniversary last week. The owners of a popular Main Line baker remain frustrated as they cry out for answers.
"I lost my son, so I don't know I'll ever have closure, but I do know that I want justice for Richard and Danielle," said Marge Petrone.
Both Petrone, then 35, and Imbo, then 34, were described as dedicated parents with nothing in their lifestyle that would suggest they could be targets of foul play.
"Somebody has to pay for this. They killed two innocent people for no reason. Danielle was a wonderful girl too, a wonderful mother," Marge said.
Petrone was described as 5'9" tall, 200 pounds with brown hair and blue eyes. He had a tattoo that read "Angela" on his left arm and one with clowns on his right arm. He was wearing a gray hooded sweatshirt, jeans and sneakers.
Imbo was described as 5'5" tall, 117 pounds, with hazel eyes, fair skin and brown hair. She had a tattoo of flowers on her lower back. She was wearing a black jacket, cream-colored sweater and blue jeans.
A $50,000 reward in the case is being offered by the Citizen's Crime Commission. If you have information, you are urged to call 215-546-TIPS (8477).
The Petrones can only hope that there's finally a break in this decade-old mystery that triggers so much agony.
"Somebody didn't just drop of out the sky and take these two people," said Marge Petrone. "This was planned."