The official, who spoke on the condition of anonymity, said a second car seemed to be traveling with Luna in Pennsylvania the night he disappeared, and credit card records suggest Luna paid for gasoline for two vehicles at a service station in King of Prussia, Pa.
They also were studying why the toll records indicate that on the night Luna died, his car was stopped at one point to get a paper toll ticket instead of gliding through the EZ Pass lane -- suggesting that someone unfamiliar with the car could have been behind the wheel.
Luna's body was found in rural Lancaster County, Pa., shortly before dawn Dec. 4. Luna, 38, had been stabbed 36 times and left facedown in a shallow creek near his still-idling Honda.
Inside the car, investigators found a large amount of blood on the floor of the back seat. They identified blood and DNA evidence from a second person, according to the law enforcement official, who said investigators think the second person could have been cut in a struggle over the knife used in the attack.
Investigators have not found a weapon. They have said Luna's wounds appear to have been from a small blade, possibly a penknife.
Authorities do not think Luna's killing was related to his work as an assistant U.S. attorney in Baltimore, where he was expected in court on the morning he disappeared to conclude a drug conspiracy trial. Instead, they suspect that his death was the result of a personal relationship and have been closely reviewing details of Luna's personal life for possible clues to his death.
Luna, a New York native who had worked as a federal prosecutor for the past four years, was married and had two young sons. In a statement yesterday, his family called the past week "extremely painful."
"The news of Jonathan's tragic and violent death has been shocking and overwhelming for us," the family said in an unsigned written statement issued to the Associated Press. "We know that all of our family, friends and Jonathan's colleagues share in our deep grief and sadness."
Officials with the Maryland U.S. attorney's office and the FBI's Baltimore field office have declined to comment on the investigation, other than to urge the public to report any information that could help solve the case. The hot line number for tips is 443-436-7772.
Investigators have issued subpoenas for records related to Luna's cell phone and his work and home computers. Some e-mail and data files on Luna's computers were encrypted, a source said yesterday, complicating the work of authorities as they try to determine whom Luna was in contact with in recent months.
It was unclear yesterday why Luna would have traveled repeatedly to the Philadelphia area. However, a key witness in the drug conspiracy case that was under way in Baltimore the week Luna was killed was being detained in that area, and lawyers connected to the case have said Luna went there to interview the witness several times.
Investigators are focused most closely, though, on Luna's final trip.
Luna had traveled from his home in Elkridge to the federal courthouse in downtown Baltimore late Dec. 3 to complete paperwork for an expected plea agreement the next morning. Authorities think he left the courthouse about 11:30 p.m. and drove toward Philadelphia willingly, stopping at least once to withdraw cash from an ATM machine.
He also is believed to have stopped at a Sunoco gas station in King of Prussia -- a busy retail area in the suburbs west of Philadelphia -- where investigators think he paid for gas for two vehicles and purchased two drinks, a soda and a bottle of water.
The Philadelphia Inquirer has reported that an attendant at the station recalled seeing Luna when he purchased the drinks about 3 a.m.. The attendant, Moustapha Balde, could not be reached yesterday. He told The Inquirer that Luna did not appear to be under any duress.
"He was just very calm," Balde told the newspaper. "He must have been with people, but I don't think he knew they were going to kill him."
Authorities yesterday were closely examining one other detail of Luna's trip, two sources said. As it traveled north, Luna's car appears to have left Interstate 95 at one point in Delaware, and when it returned to the highway, the driver stopped to get a paper ticket at a toll booth instead of driving through the EZ Pass lane.
The sources said that the stop could indicate that someone other than Luna was behind the wheel, unaware that the vehicle had an EZ Pass card.
Sun staff writers Laura Sullivan, Lynn Anderson and Gus G. Sentementes contributed to this article.