Yvette Perez told a Philadelphia jury Wednesday that Howard Cain had been coming by the bank and flirting with her for months before the May 2008 robbery in the city's Port Richmond section.
Authorities say Cain - who was later killed in a shootout with police - fatally shot Sgt. Stephen Liczbinski while Cain and two accomplices were trying to get away.
The men who allegedly participated in the robbery with Cain are now on trial in the officer's death.
Thirty-five year-old Eric Floyd and 41-year-old Levon Warner are charged with first-degree murder.
Prosecutors are seeking the death penalty.
