Pastor Rick Warren said his son killed himself with an unregistered gun he purchased through the Internet.

Warren sent a tweet Thursday saying he forgives whoever sold the weapon to his 27-year-old son Matthew, who committed suicide last Friday.

The Orange County Sheriff's Department is trying to find the seller but it won't be easy. The gun's serial number was scratched off, making it impossible to trace, spokesman Jim Amormino said.

"We can't tell if it's registered or not because the serial number is scratched off," he said. "At one point in time, it may have been, but it's going to be impossible to find out."

It's illegal in California to buy a gun without a background check and purchasers are supposed to register their firearms. Defacing a gun's serial number is a federal offense.

Rick Warren is pastor of Saddleback Church in Orange County and author of "The Purpose-Driven Life," a bestseller.

He and his wife, Kay, said in an email on Saturday that their son struggled for much of his life with severe depression and suicidal thoughts. They have set up a mental health fund in Matthew's memory.

Warren has tweeted daily since his son's death, including items about seeing his son's body at the coroner's office for the first time, combating the stigma of mental illness and the challenges of grieving when a public figure.

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