NEW YORK, NY.- On Thursday evening, the doors abruptly closed at the Isabella Stewart Gardner Museum in Boston. Officials had learned that climate protesters were planning a visit during the hours when the cultural institution offers free admission.

Activist group Extinction Rebellion had posted on social media earlier in the day, saying this would be its second attempt at visiting the museum. “This is a peaceful field trip without the risk of arrest,” the invitation said.

In March, demonstrators had tried to stage a “guerrilla art installation” that would have involved inserting their own images into empty picture frames at the museum, an action intended to draw attention to the loss of biodiversity. But the event also would have fallen on the same day as the infamous art heist at the Gardner 33 years earlier, and executives were nervous about security risks and decided to close the museum. Protesters instead carried flags and red banners, staging a “die-in” near the museum’s entrance.

On Thursday, the museum’s director, Peggy Fogelman, wrote a public note to explain the second sudden closure in six months. “These frames are not only important and fragile historic objects in their own right, but they memorialize the tragic 1990 theft that deprived our public of the opportunity to enjoy unique masterpieces by Rembrandt, Vermeer and others,” she wrote. “It is heartbreaking to associate the painful reminder of this loss with any scenario that would jeopardize the frames themselves or the experience of our staff and visitors.”

For more than a year, climate protesters have targeted museums as a method of gaining attention for their cause. One of the latest attacks occurred at the National Gallery of Canada in Ottawa, Ontario, where a protester threw pink paint on Tom Thomson’s 1915 painting “Northern River.” The museum said the artwork was unharmed during the incident thanks to a protective glazed panel installed on the canvas.

Some of the most serious charges against demonstrators stemmed from an episode in April, when two activists splattered paint on the case surrounding a 19th-century Degas sculpture at the National Gallery of Art in Washington. Because the museum is a federal institution, the protesters are now facing federal charges, including conspiracy to commit an offense against the United States.

During an interview, Fogelman said the Gardner Museum needed to increase its spending on security because of the ongoing threat of protests; it also lost out on revenues generated by the 1,300 people who usually visit the museum on a Thursday evening, spending money in the gift shop and restaurant.

“The most painful part of the decision was that we had to curtail our free hours,” Fogelman said. “It deprives our community of the chance to really immerse themselves in the experience of art at the Gardner Museum.”

She questioned why protesters would target the museum, which dwells inside a building designed to have a low carbon footprint by drawing its energy from geothermal power. The museum also maintains a renowned garden and currently has an exhibition called “Presence of Plants in Contemporary Art,” reflecting the closeness of artists with the natural world.

“The Gardner Museum simply serves as a conversation-starter,” said Jamie McGonagill, the media and messaging director for Extinction Rebellion’s Boston branch. She said the activists were planning to wear shirts with the images they had originally wanted to insert into the empty frames. “There was no civil disobedience planned. There was no disruption of guests planned.”

This article originally appeared in The New York Times.