LAWRENCE (CBS) — Surveillance video that Lawrence police released last week may have helped catch a killer.

The video was of a man wanted in connection with a murder last summer.

Rafael Martinez, 19, of Lowell, was in court Monday on charges he murdered Timothy Walker, 20, of Lawrence. Martinez was ordered held without bail.

Martinez was arrested days after the video was released.

Prosecutors say Martinez shot and killed Walker on Walker’s front porch on Lowell Streeton July 24, 2010. Investigators say they are still unsure of the motive of the shooting, but believe Walker was the intended target.

In court Monday,  Martinez also pleaded not guilty to charges of illegal possession of a firearm and illegal possession of ammunition.