A federal law enforcement official, speaking on condition of anonymity, said authorities have not established a motive for the slaying — the first killing of a federal prosecutor in two years.
Investigators are interviewing people connected with cases Luna prosecuted, as well as friends and associates, but no immediate promising leads have come up, the official said.
Luna apparently was attacked after leaving his office in Baltimore around midnight Wednesday, the source said. His body was discovered six hours later and 70 miles away, near his blood-smeared, idling car, according to a police affidavit.
Lancaster County, Pa., coroner Dr. Barry Walp said the 38-year-old assistant U.S. attorney was "brutalized with multiple stab wounds" that could have been caused by a penknife, and then drowned in the creek.
"They were defensive wounds," a second federal law enforcement source told The Associated Press on condition of anonymity.
Walp said Luna was dressed in a suit and overcoat, and had his wallet with identification and cash, but it was unclear whether he had been robbed.
Money and cell phone equipment also were found inside his car, which had blood on the driver's side door and fender and a large pool of blood on the floor, according to a police search warrant application. The affidavit said Luna also had a "traumatic wound" on the right side of his head.
The FBI worked to create a timeline of what Luna did in his last hours.
By 5 p.m. Wednesday, Luna and defense attorneys had reached a plea bargain in the case of rap musician Deon L. Smith and Walter O. Poindexter, who were on trial on charges of running a violent heroin ring from their studio, according to the judge presiding over the case.
Poindexter's attorney, Arcangelo Tuminelli, said he got a call from Luna at 9:06 p.m. in which the prosecutor said he was still drawing up the paperwork for the plea and making sure it was all correct.
Tuminelli said he did not where Luna was at that point. But he said Luna told him he had to go home and would be back in his office in the federal courthouse in Baltimore later.
"I assumed there would be a fax at my house of the agreement by about midnight," Tuminelli said. The fax never came.
Authorities didn't say whether the case had anything to do with the slaying. Smith and Poindexter were behind bars at the time. Smith's attorney, Kenneth Ravenell, says it would make no sense for his client or Poindexter to be involved.
"The deal that Mr. Smith pled guilty to was a deal we offered to the prosecutors two weeks before the trial," he told CBS News Early Show co-anchor Hannah Storm. "Mr. Luna was instrumental in negotiating that deal and convincing his office that it was a proper deal. So both men had the deal that they had sought from the prosecution."
One federal law enforcement official said authorities had determined Luna left his home early in the evening and went back to his office to work on papers in the plea bargain. He was there until around midnight, the source said.
Another source who spoke on condition of anonymity said there were personal items in Luna's office that one "would have expected him to be taking home had he been leaving to go home for the evening." The source declined to describe the items.
Authorities have not said whether the rapper's case had anything to do with the slaying. Smith and Poindexter were behind bars at the time.
Tuminelli said the FBI interviewed Poindexter on Thursday night.
"He had absolutely no information that would be of help to them," he said. "I believe that this has nothing to do with my client or Mr. Smith."
Tuminelli said Smith also consented to be interviewed by authorities.
The last federal prosecutor to be slain was Assistant U.S. Attorney Thomas C. Wales, who was shot to death in Seattle in October 2001. That slaying remains unsolved.
Attorney General John Ashcroft called it a "tragic death."
"I express our deepest condolences to Jonathan's family, colleagues and friends," Ashcroft said. "We share his family's grief and will provide any support and assistance to help them through this difficult time."