Three years ago today, Richard Petrone and Danielle Imbo left a South Philadelphia bar and were never seen by friends and family again.

Unlike many missing-persons cases, this one has lived up to the name of the TV show Without a Trace.

The very lack of evidence, though, fits the theory announced earlier this month by the FBI:

The pair were victims of a professional hit.

Investigators hope that tips will help them catch the culprits.

"This is one of those cases that kind of haunts you," recalled Sgt. Tim Cooney, who was part of the city's investigation in 2005. ". . . If it was a random act, there's usually some evidence, whether it's a car left behind, or, God forbid, a victim, or a handbag throw aside."

Witnesses said the pair seemed happy as they left Abilene's bar and restaurant on South Street just before midnight and headed for her home in Mount Laurel.

Investigators found no sign that they ever arrived.

Besides their residences, "there were waterways checked, there were woods and wooded lands checked, highways were checked - it was pretty extensive on both sides of the river," Cooney said.

Family members also conducted their own searches, even following suggestions from psychics.

But nothing definitive was found.

The lack of evidence is partly why the FBI suspects a case of murder for hire.

But new tips and leads have also proved "promising," FBI spokeswoman Jerri Williams said this morning.

She declined to get into specifics, such as names of possible suspects, but said that alternative scenarios are unlikely.

Kidnappers, for example, leave notes or send messages.

Thieves use credit cards. Carjackers abandon cars.

The victims' phones and credit cards were never used, and Petrone's black Dodge Dakota pickup - Pennsylvania license plate YFH 2319 - has never been found.

Could either party have gotten violent and then disappeared?

"There is nothing that would indicate that that would be the case," Williams said.

Investigators are appealing for more clues to crack the case.

"No one just disappears," Williams said. "There are individuals out there that know what happened or have heard what happened, and we're just trying to reach out to those individuals and ask them to come forward."

Philadelphia homicide detectives, the Mount Laurel police and the New Jersey State Police are also investigating.

A $100,000 reward for information on the couple's whereabouts is still being offered through the Citizens Crime Commission.

Anyone with information about the case is asked to call the FBI at 215-418-4000 or the Citizens Crime Commission at 215-546-8477.

More information about the case can be found at www.danielleimbo.com and www.richardpetrone.com.

Contact staff writer Peter Mucha at 215-854-4342 or pmucha@phillynews.com.