12:55 p.m. update:
All lanes of southbound 295 have reopened, according to the D.C. police. Watch for lingering delays.
D.C. police have shut down all lanes of southbound Interstate 295 at exit 1 near Overlook Avenue. Police say they are investigating a suspicious vehicle and the explosive ordnance division is responding.
John Lisle, spokesman for DC Water, tweeted from the scene: