UPDATE: BART tweeted at 6:20 a.m. Thursday that Montgomery BART station has reopened. There are still residual 20-minute systemwide delays.
BART officials reported early Thursday that the Montgomery Street Station in San Francisco was closed due to a medical emergency.
Muni is providing bus service for Downtown stations until the situation is resolved. No estimate for the reopening of the station was provided in a 5:50 a.m. tweet from BART.