In the ER the medics call the time of death of the hit woman as 16.42. A few minutes later Harper and Thorsen are seen via their shop-cam and the time is 15.24.
The Major Crimes division of the L.A. County D.A. (the division that would definitely be handling a major RICO case) is housed in the Hall of Justice, along with the L.A. County Sheriff's admin offices. This would be a secure-access building. There's no way the hit man would have been able to gain entry, especially after-hours, and definitely not with a weapon, as entering the building would require passing through metal detector and x-ray screening.