From its grim beginning in a rural Pennsylvania field five weeks ago, the mystery of who killed Baltimore federal prosecutor Jonathan P. Luna has only deepened as initially promising leads have soured and potential evidence troves have failed to identify a suspect.

Privately, investigators have expressed frustration that their efforts have yet to produce a break in the high-profile case. Agents again retraced Luna's final movements this week and visited a Pennsylvania Turnpike tollbooth to ascertain how well workers can see into the backs of vehicles. A source close to the investigation called those steps "desperation stuff."

The source, a law enforcement official who spoke on condition of anonymity, said there has been little clear progress and some setbacks in the case in recent weeks. Most significant, though authorities collected DNA and partial fingerprint evidence, they have not matched those clues to a potential suspect.

Authorities have been able to make only limited use of video surveillance cameras along the route that Luna traveled from Baltimore, through Delaware and toward Philadelphia, on the night he was killed, the source said. For instance, investigators know that Luna's bank card was used to withdraw $200 at a rest stop in Newark, Del., but there is no videotape capturing that transaction.

At the Sunoco station in King of Prussia, Pa., where Luna's credit card was used a few hours later, there is no sign of him on the station's grainy video surveillance tapes, the source said. Investigators are trying to determine whether another man who can be seen on the tape could have been traveling with Luna at that point.

The well-liked and energetic lawyer was found dead shortly before dawn Dec. 4 in rural Lancaster County, Pa., about 50 miles west and less than three hours after the gas station stop. Luna had been stabbed 36 times and left facedown in a small creek, where he drowned. His blood-smeared Honda Accord was nearby, its engine running.

In the first weeks of the investigation, the killing drew widespread media attention and inspired far-reaching theories by armchair and Internet detectives.

The 38-year-old prosecutor had disappeared as he was preparing to conclude a drug conspiracy trial in U.S. District Court in Baltimore, but authorities have found no evidence linking the killing to Luna's work and instead have closely reviewed details of Luna's personal life for possible clues to explain his mysterious death.

Over the past month, agents have pored over Luna's financial records and computer files, and combed through his phone logs and entries in his Palm Pilot, but none of the information has pointed to a culprit in the death of Luna, a married father with two young sons.

Publicly, authorities remain tight-lipped about the case.

Luna's boss, Maryland U.S. Attorney Thomas M. DiBiagio, has not commented on the investigation since the day it began. The Baltimore FBI office, which is conducting the investigation with state police in Pennsylvania and Delaware, urges anyone with information to call investigators at 410-265-8080.

"The FBI continues working with our state, local and federal counterparts, running out every available lead," Special Agent Barry M. Maddox, a spokesman for the Baltimore field office, said yesterday. "We're still continuing to work all leads very aggressively."

One central challenge for investigators is time. Jack Levin, a criminologist at Northeastern University in Boston, said in an interview yesterday that of the roughly 60 percent of homicides in the country that are solved each year, most are solved within 48 hours of the crime.

After that, "the case gets colder and colder with every passing week," said Levin, who is not directly involved in the Luna investigation.

"Here, it sounds like they've got physical evidence that sooner or later may lead to the killer," he said. "At this point, however, often what is required is a stroke of luck. ... Maybe an eyewitness comes forward, or the killer has a pang of conscience and confesses."

As the investigation continues, local authorities have been pulled in other directions.

In typically quiet Lancaster County, Pa., the driver of a Jeep Cherokee and four passengers -- all younger than age 20 -- were killed when the Jeep hit a snowplow head-on during a snowy weekend in mid-December. On Christmas Day, three members of a family were killed when the lights on their Christmas tree started a fire in their duplex home.

More recently, a newborn girl, her umbilical cord attached, was found dead in a trash barrel outside an area Amish school.

But for local law enforcement officials such as Edward Karcher, chief of police in the tiny township where Luna's body was found, the Luna case remains fresh in his mind. Although Karcher has not played a central role in the investigation, he said he is confident that the killer will be found.

"Some people think that just because they don't see anything in the newspapers, the case has gone cold," Karcher said. "But [police] are working on it. ... I think this is a very solvable case. It just takes time. It's just a matter of time."