Streets near Third and Mission streets were briefly closed as police investigated the incident.
The streets are slated to open soon, police spokesman Officer Albie Esparza said on Twitter at 3:07 p.m.
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Esparza initially reported the incident on Twitter at 2:39 p.m.
San Francisco Municipal Railway buses were affected by the investigation, according to Muni officials.
Inbound Muni buses on the 8-Bayshore line and outbound Muni buses on the 30-Stockton and 45-Union/Stockton lines were affected.
No other details about the incident were immediately available.