The San Francisco Zoo temporarily closed on Monday after receiving an unsubstantiated threat from an anonymous caller, officials said.
The zoo shut down at about 2:50 p.m. after officials received the threat, and San Francisco police arrived a short time later to investigate.
During the response, about 400 to 500 visitors and zoo staff were directed to shelter in place inside secure buildings across the zoo grounds, while officers conducted a sweep.
By about 3:35 p.m., police said they had completed a thorough search and found no evidence of a credible threat. The shelter-in-place order was lifted, and the zoo reopened for the remainder of the day.
Zoo officials said the threat follows a series of swatting and similar incidents at zoos around the country, including bomb threats at the Central Florida Zoo and Botanical Gardens on June 7, the Como Park Zoo and Conservatory in St. Paul, Minn. on June 6 and the Toledo Zoo in Ohio on May 31.
The incidents prompted evacuations and police sweeps, but in each case authorities found no credible threat, according to local reports.