Zoo Miami was targeted by another apparent swatting incident Monday, the fourth straight day authorities responded to bomb threats or suspected hoax calls at the popular South Florida attraction.

Local station WTVJ reported that investigators were at the zoo after a possible swatting call prompted a law enforcement response. The Miami-Dade Sheriff’s Office said specialized units conducted a sweep of the property, though officials indicated the threat appeared to be a hoax.

The string of incidents began Friday when the zoo was forced to close while deputies searched the property following a bomb threat. Authorities evacuated visitors and staff as law enforcement combed the grounds.

A second threat on Saturday triggered another evacuation and search, and a third incident was reported Sunday, according to WTVJ and statements from the zoo.

Zoo Miami said deputies, including K-9 units that were already on site, conducted a full sweep Sunday before issuing an all-clear. The zoo later announced plans to reopen Monday with enhanced security measures in place.

Authorities have not found explosives during any of the searches so far. Investigators believe the calls are part of a swatting campaign, a tactic in which false emergency reports are made to trigger large police responses.

Visitors returning to the zoo Monday reportedly encountered an increased law enforcement presence, including K-9 units patrolling the property.

Zoo Miami, operated by Miami-Dade County, is one of the largest zoological parks in the United States. Located in southwest Miami-Dade, the facility houses thousands of animals and is known as the country’s only mainland subtropical zoo.