A man believed to have illegally taken prescription medication was arrested Tuesday an accused of causing a head-on crash that killed the other driver.

William Jacob Western, 47, was arrested for investigation of negligently operating a vehicle resulting in death, reckless driving and two counts of unlawful possession of a prescription drug.

The crash occurred about 8 a.m. near the intersection of Sunset Boulevard and Valley View Drive. Multiple witnesses observed Western "driving erratically on the road, speeding and ultimately leaving (his) lane of travel, causing a collision in the center yellow lanes," according to a police booking affidavit.

Western's pickup truck collided with a pickup driven by Manuel Gomez, 55, who later died as a result of his injuries, according to police.

An officer met with Western at the crash scene.

"I noticed his pupils to be pin-prick constricted, his eyes were bloodshot and watery and he was sweating on his face and the top of his shaved head. William was making small fidgeting movements with his hands and feet, and was unable to sit still," the affidavit states.

Field soberly tests were administered, which Western performed poorly, according to police.

"William admitted to using his Adderall prescription prior to the crash, and a hydrocodone pill obtained illegally the night prior. William said that he also used an Ambien to fall asleep, which was a new prescription for him," the affidavit alleges.

Two pills were found in Western's pocket, neither "in their proper container, or labeled appropriately," police said.

Western was arrested following the conclusion of the tests.

Contributing: Pat Reavy