Image credit: Jae C. Hong/AP Photo

Pastor Rick Warren said Thursday that his son, Matthew, purchased an unregistered gun over the Internet. It was that weapon that he used to take his own life last week after years of battling mental illness.

"Someone on the internet sold Matthew an unregistered gun. I pray he seeks God's forgiveness. I forgive him. #MATTHEW 6:15," Warren tweeted Thursday.

The Orange County Sheriff's Department told the Associated Press it is trying to identify the seller, but the gun's serial number was unreadable, making it difficult to trace the firearm.

"We can't tell if it's registered or not because the serial number is scratched off," Jim Amormino, spokesman for the Orange County Sheriff's Department, told the AP. "At one point in time, it may have been [registered], but it's going to be impossible to find out."

California requires background checks on all gun purchases, and it is a federal offense to deface a firearm's serial number.

An autopsy revealed Matthew Warren died of a self-inflicted gunshot wound last Friday. On Saturday, the Warren family sent a message about their son's death to the Saddleback Church community, where Rick Warren has been pastor for over 30 years.

"Our youngest son, Matthew, age 27, and a lifelong member of Saddleback, died today," Rick Warren wrote. "Only those closest knew that he struggled from birth with mental illness, dark holes of depression, and even suicidal thoughts. In spite of America's best doctors, meds, counselors, and prayers for healing, the torture of mental illness never subsided. Today, after a fun evening together with Kay and me, in a momentary wave of despair at his home, he took his life."