The L.A. County District Attorney's office announced the murder charge against Shawn Laval Smith Friday, along with a special allegation that he used a deadly and dangerous weapon, a knife.

As the charges were announced, D.A George Gascon said, "Those who show no compassion for human life will face serious consequences. The murder of Brianna Kupfer has left Los Angeles County devastated and my office is reaching out to her family to provide any services they may need.”

TMZ broke the story ... Laval was arrested Wednesday in Pasadena after a city-wide search following Brianna's murder. Laval allegedly walked into the Hancock Park furniture store where Kupfer was working on January 13 and stabbed her to death.

Cops were quick to put out photos and surveillance of Smith -- taken at a 7-Eleven store shortly after the murder -- urging the city to help track him down.

We obtained video of Smith's arrest from Wednesday, we're told a man called police after believing he spotted Smith walking the streets ... turns out, he was right.