Investigators are conducting DNA analysis on the blood, the official said, speaking on the condition of anonymity. The officials would not say how much blood was found or whether police had any clues to the killer's identity.
If two sources of blood are identified, it could suggest that Luna struggled with his killer, but officials cautioned that a second source of blood could be from an unrelated event.
Luna, an assistant U.S. attorney in Baltimore, was found last week, stabbed and drowned in a creek in rural Lancaster County, Pa., about 70 miles from his suburban Baltimore home.
According to a police affidavit, investigators found a pool of blood on the floor of the back seat of Luna's car, which was idling near the creek where he was found.
The FBI has said it submitted evidence for forensic testing but would not say whether that testing had been completed.
Authorities previously said some of Luna's 36 stab wounds appeared to be defensive in nature, though Lancaster County Coroner Barry Walp said he did not observe any defensive wounds during the autopsy.
Defensive wounds are typically found on the hands and arms. Walp said Luna's wounds were all in the neck and upper chest.
Authorities also have said Luna's 2003 Honda Accord was the only car involved in his roundabout trip from Baltimore to Lancaster County, despite accounts from gas station workers that Luna bought gas for his and another vehicle the morning he was killed.
Investigators would not comment on how the killer might have escaped the crime scene.
Family members and friends said they believe Luna's death is connected to his job. The 38-year-old had prosecuted drug dealers, armed bank robbers and child molesters during his career.
Investigators have found no evidence of such a link and have been probing Luna's personal life, including asking questions about whether he had a girlfriend or financial problems.