An 18-year veteran of the San Francisco Police Department surrendered to officers Tuesday, after police say a year-long investigation revealed that he had manufactured and was in possession of a weapon prohibited by California law.

The San Francisco Police Department's Internal Affairs investigation into the activities of 50-year-old Officer Thomas Abrahamsen began in the summer of 2015, SFPD spokesperson Sergeant Michael Andraychak says, after "other Department members" tipped them off.

The year-long investigation, Andraychak says, revealed that Abrahamsen "manufactured and was in possession of a prohibited AR-15 style assault rifle and AR-15 receiver components."

California's Roberti-Roos Assault Weapons Control Act of 1989 banned Colt brand AR-15 rifles. California's Assault Weapons ban, which was enacted in 2000, further banned possession of AR-15 assault weapons that weren't owned and registered when the law took effect.

According to the CA Attorney General's Assault Weapons Identification Guide:

Any firearm of minor variation of the AK or AR-15 type (i.e., series weapon), regardless of the manufacturer, is a Category 2 (Kasler v. Lockyer) assault weapon under the original Roberti-Roos Assault Weapons Control Act of 1989.

All AK and AR-15 series weapons had to be possessed before August 16, 2000 and must have been registered on or before January 23, 2001. The Department of Justice is required to identify these series weapons and includes in this publication a listing of dentified AK and AR-15 series weapons.

It is important to note that removal of a firearm’s characteristics does not affect its status as a Category 2 assault weapon. A Category 2 assault weapon is still an assault weapon even if it has no Category 3 (SB 23 - generic characteristics) features. Category 2 assault weapons may be of any caliber, including .22 caliber rimfire.

Andraychak says that Abrahamsen, a resident of Berkeley, surrendered to the authorities Tuesday, and was booked into San Francisco County Jail " for one felony count of manufacture of an assault weapon in violation of 30600(a)PC and one felony count of possession of an assault weapon in violation of 30605(a)PC."

Abrahamsen has also been placed on unpaid administrative leave, Andraychak says.

According to a spokesperson with the San Francisco Sheriff's Department, as of Wednesday morning Abrahamsen remains in custody. The completed investigation is now in the hands of San Francisco's District Attorney's Office, Andraychak says, which will make the final determination on what charges Abrahamsen will face in court.