Jasmine Jaw, a frustrated 27-year-old from San Jose, California, had already bought tickets. She knocked on the massive wooden double doors Tuesday morning and tried peering in when she got no answer.
"We wanted to see this," she said. "We've come all this way."
"It's definitely disappointing," said Kaustubh Girme, a 30-year-old from Boston who was visiting Puerto Rico with his wife. "It's really frustrating that ordinary people have to suffer for something they couldn't do."
U.S. Forest Service Agent Carmelo Ortiz said he turned away numerous tourists trying to visit El Yunque on Tuesday.
The National Guard of Puerto Rico said more than 600 of its 800-strong force are not working, along with more than 100 federal government contractors.
In the neighboring U.S. Virgin Islands, nearly 700 federal employees who work there have been affected, according to Congressional Delegate Donna Christensen.
Among those still on the job were some 180 air traffic controllers, more than 600 U.S. Customs and Border Protection agents and employees at existing Head Start centers in Puerto Rico.