WEST SACRAMENTO, Calif. — As darkness falls, the most tattered section of this town’s main drag feels more desperate with each passing hour. Under the cover of night, a slow but steady flow of wandering souls emerges — addicts, prostitutes, drifters. Sergio Alvarez knew the pickings were easy.

As a rookie officer on the West Sacramento police force, Alvarez was assigned to the overnight shift on a beat that included West Capitol Avenue. He volunteered to stay on late-night duty and, over his nearly six years on the job, he gained seniority and almost always patrolled alone. With the solitude came opportunity.

“That’s where Alvarez falls through the cracks,” said Sacramento attorney Justin Gingery, whose firm represented four of eight women who said they were sexually assaulted by the officer, many in a dark alley near “West Cap.” Convicted last year of kidnapping five of those women and either raping them or forcing them to perform oral sex, Alvarez is now serving 205 years to life in prison.

Alvarez is a poster child of a predator cop — and of the flaws in policies, technological glitches, and culture of policing that can allow such behavior to go unnoticed or unpunished until it’s too late. His case prompted civil lawsuits over police procedures, with a total of $4.1 million to be paid to six victims who sued, and left a new chief taking a hard look at the way the department does business.

“It hurts the heart to see victims. But it makes it even worse when you are, in one way, shape or form, a contributing factor to them being hurt,” said Tom McDonald, a former captain for the Los Angeles Police Department who took over in West Sacramento after Alvarez’s arrest.