ALERT DESK: LA Threat Fizzles Out, Roof Trial Begins, Possible Syrian Cease-Fire

Today at the Alert Desk:

- It was released on Monday of a threat which targeted Los Angeles's transit system but the FBI says there is no evidence that the threat was credible.

- The murder trial for Dylann Roof, the accused Charleston, South Carolina church shooter, begins today.

- Rebel factions in Syria proposed a five-day cease-fire for medical and civilian evacuation from Aleppo.