The defense rested Monday in the murder trial of former New England Patriots player Aaron Hernandez after a doctor testified that PCP can cause people to be suddenly violent, even days after smoking it.

Hernandez has pleaded not guilty in the June 17, 2013, killing of Odin Lloyd, who was shot six times at an industrial park less than a mile from Hernandez's home in North Attleborough. Lloyd was dating the sister of Hernandez's fiancee. At the time of the killing, Hernandez had a $40 million contract with the Patriots.

Hernandez lawyer James Sultan called just three witnesses.

The first was Dr. David Greenblatt, a professor at Tufts University School of Medicine, who testified about the effects of the drug PCP. Hernandez's cousin testified that his two co-defendants, Ernest Wallace and Carlos Ortiz, were smoking what she thought was PCP the weekend the crime occurred. Both men have pleaded not guilty and will be tried later.

Prosecutor Patrick Bomberg went after Greenblatt's testimony, showing surveillance video of Ortiz, Wallace and Hernandez from Hernandez's home before and immediately after the killing. In the videos, the men do not appear to be acting strangely.

"This is essentially three minutes after the murder — you'd expect to see something lingering of PCP psychosis or intoxication," Bomberg said, asking if Greenblatt could detect such behavior.

"You just can't tell," Greenblatt replied.

After the defense rested, prosecutors then called a rebuttal witness to testify about PCP. That witness, Dr. Martin Brecher, a psychiatrist, said it was wrong to associate PCP and violence. Brecher wrote a paper reviewing cases of PCP over decades.

After the prosecutor finished questioning Brecher, the judge called for a recess and sent the jury out of the room.

Testimony earlier Monday by two other witnesses related to DNA on a shell casing found inside a silver Nissan Altima that Hernandez rented, which prosecutors say he used to drive Lloyd to his death. When police found the shell casing in a trash bin at a rental car agency, it was stuck to chewed blue bubble gum that also was found in the Nissan. In testing, Hernandez's DNA was found on the shell casing. The defense has argued it got there from the bubble gum.

The judge has said she expects closing arguments to be delivered Tuesday or Wednesday.