Police Chief David Snowden also announced at a press conference that Garrison had been driving in excess of the 25 mph speed limit and was under the influence of cocaine at the time of the accident. According to toxicology reports, Garrison's blood alcohol level was at 2.0 percent, two and half times the legal limit.
The District Attorney's office now must decide whether to formally charge the actor.
The incident occurred in Beverly Hills last December 2, when the Land Rover that Garrison was driving jumped a curb and struck a tree, killing 17-year-old passenger Vahagn Setian. The accident also injured two 15-year-old girls who were in the car, according to police.
Garrison had met Setian and the two teenage girls earlier at a party. Witnesses claim the saw Garrison drinking large amounts of vodka at the party.
Garrison's high-powered attorney, Harlan Braun, couldn't be reached for comment, but a source in his office tells Usmagazine.com, "He won't say anything until he has a chance to read the reports and let the DA evaluate the case."
If convicted of the manslaughter charge alone, Garrison could serve up to 10 years in prison.