Halifax Regional Police are asking the public for help in tracking down a key witness in a murder trial. 

A Canada-wide warrant for witness, as it's known, was issued last week for Walter Madeaus Brooks.

Brooks is a witness in the second-degree murder trial of Demarqus Shane Beals, who is charged in the 2011 stabbing death of Keya Simon.

Simon, a 19-year-old mother, was fatally stabbed at a party at her sister's apartment on Pinecrest Drive in north-end Dartmouth. Her sister was also stabbed but survived. 

A jury was picked last week for the trial in Nova Scotia Supreme Court but the case has yet to proceed.

It wasn't until August 2014 that Beals was charged. He's been free on bail awaiting trial.

Police say they believe Brooks, who is originally from Halifax, is living in Orangeville, Ont.