Meki Gaono told investigators he wasn't thinking when he pulled the trigger, killing Oceanside police Officer Dan Bessant in December 2006.
Prosecutors said that Gaono, 17 at the time and a documented gang member, fired the fatal shot that killed Officer Bessant during a routine traffic stop.
Authorities have concluded the killing was done for no other reason than it presented a chance for three documented gang members to gain respect, however warped, for shooting at police.
Gaono, now 20, is on trial for the shooting and faces life in prison if convicted.
Last week, jurors heard portions of Gaono's first interview when he said he was playing basketball with friends at the time of the shooting.
Prosecutors said Gaono and two fellow teenage gang members had been drinking and playing with guns when they spotted police at a traffic stop and shot at the officers.
During questioning, a police officer asked Gaono what he would tell Bessant if he could and supplied the teenager with a pencil and paper, according to our media partner the North County Times.
"I was under the influence that night, and if I could take it back, I would," Gaono said in an apology letter he wrote to Oceanside police Officer Dan Bessant near the end of an all-night interrogation. "Please find it in your heart to forgive me."
At that point, he did not know the 25-year-old Bessant was dead, reports Teri Figueroa with our media partner the North County Times.
Last year, Penifoti Taeotui was convicted of murder in the first degree for his role in the shooting. He has been sentenced to life in prison.
Charges against the third suspected gunman were dropped because prosecutors didn't have enough evidence, but he remains under investigation.
Read more about the trial on our media partner's website, The North County Times.