In court documents released Tuesday, Santa Monica police Detective Derek Leone said Howell had a three-fifths-full 25-pound container of Shoc-Shot, a commercially available "binary explosive," on the front passenger floorboard.
"The amount of explosives in the container would have posted a grave danger to both persons and property had the explosives been detonated, either intentionally or accidentally, in a public space," Leone wrote in a statement supporting prosecutors' request to increase Howell's bail amount.
Howell had three rifles in the car, Leone wrote, including an Anderson Manufacturing AM-15 .223-caliber rifle loaded with a 30-round detachable magazine with a second 30-round magazine taped to it.
He also had a loaded .30-06-caliber bolt-action rifle and a .22-caliber Ruger semi-automatic rifle, according to the detective's statement.
The weapons were believed to have been purchased by Howell's father, the prosecutor said in court, citing a federal trace of the firearms.
Rounds from either of the first two rifles could have detonated the explosive, Leone said.
Howell also had a Taser, handcuffs, a buck knife, a security badge, a black hood, more ammunition and a 5-gallon container of gasoline.