I live and work relatively near the Federal Courthouse and it now seems likely that the sirens I heard were in response to a deadly shooting that occurred shortly after 8 AM this morning. What appears to have been a single man acting alone entered the building and began firing shots. A court security officer was shot and killed and a US Marshall wounded before the gunman attempted to flee. He was shot in the head and killed outside the courthouse.
In addition to the Federal Courts, the building houses offices for several other agencies and offices for both Nevada Senators, Harry Reid (D) and John Ensign (R). Neither man was in the building at the time of the incident.
The motive of the shooter is still unknown.