A man reported to have threatened a porn shop clerk with a knife Thursday "continued to advance" on a San Diego police officer moments before the officer shot and killed him, according to police officials.

Several evidence markers were placed in the alley outside a strip clup and adult bookstore on Hancock Street at Camino Del Rio in the Midway District.

San Diego Police homicide detectives searched the alley and even looked through a dumpster as they collected evidence in the deadly officer-involved shooting that took place a few minutes after midnight.

One investigator visited the company next door that has a surveillance recording system that covers the area.

According to police, someone reported seeing a man with a knife threatening the clerk from an adult bookstore.

The arriving officer saw a man in the alley matching the suspect description and gave the man some verbal commands, officials said.

When the unidentified man, described by officials as 42 years old, did not comply, he was shot, officials said.

Paramedics were seen giving CPR to the suspect. He was brought to UCSD Medical Center where he was declared dead.

Detectives did not reveal what was said or why the officer, a 27-year veteran, decided to open fire.

There were two to three small markers on the ground that investigators usually use to mark bullet shell casings.

We don't know much about the suspect but police say they will reveal more as the investigation continues.

A clerk was taking a smoke break in the alley when he saw what looked like transient wielding a knife saying he was going to kill people, according to a co-worker.

Store employee Chris Rapata told NBC 7 that interaction is what prompted the clerk to call 911.

The bookstore is located next to the World Famous Body Shop strip club, west of Interstate 5 and north of the Rosecrans Street and Sports Arena Boulevard intersection.