DENVER (AP) — The U.S. Interior Department said Friday it will go ahead with plans to open a wildlife refuge at the site of a former nuclear weapons plant in Colorado, after briefly putting the opening on hold amid concerns about public safety.

Rocky Flats National Wildlife Refuge, on the perimeter of a government factory that made plutonium triggers for nuclear bombs, is scheduled to open Saturday.

Interior Secretary Ryan Zinke initially said Friday he would delay the opening to gather more information about safety. The announcement came after Colorado Democratic congressman Jared Polis, who is running for governor, wrote Zinke expressing concerns that plutonium testing on the site was outdated and asking him to postpone the opening until new tests could be done.

Just one hour later, Zinke spokeswoman Faith Vander Voort said a review was complete and the refuge would open.

Vander Voort said the review was done by Deputy Secretary David Bernhardt, the No. 2 leader at the department. Vander Voort did not provide any details of the review and did not immediately respond to an email seeking more information.

The Rocky Flats plutonium plant stopped work in 1989 after a 34-year history marred by fires, leaks and spills. It was shut down during a criminal investigation into environmental violations.

Rockwell International, the contractor then operating the plant, pleaded guilty in 1992 to charges that included allowing leaks of chemical and radioactive material and illegally disposing hazardous waste. The company was fined $18.5 million.

The plutonium plant was cleaned up at a cost of $7 billion, but it remains off-limits to the public. The 8-square-mile (21-kilometer) buffer zone surrounding the manufacturing site was turned over to the U.S. Fish and Wildlife Service for a refuge.

Some groups worry that plutonium particles eluded the cleanup and could be sprinkled over the refuge, where hikers and cyclists could inadvertently stir them up or track them home.

Five environmental and community activist groups sued the government in May, arguing the refuge should remain closed until more testing is done.

Last month, a judge rejected their request to delay the opening while the lawsuit is heard. The lawsuit is pending in Denver federal court.

"My head is spinning," said Randall Weiner, an attorney for the plaintiffs, after the Interior Department's rapid reversal Friday.

"It seems like the (deputy) secretary did an awfully quick study to address the questions raised by Rep. Polis," he said.

Until this weekend, the only way to visit the refuge was to sign up for a short hike, guided by a Fish and Wildlife Service officer, offered once a month.

The agency plans to open about 10 miles (16 kilometers) of trails this weekend that will be open seven days a week. Visitors will be told to stay on the trails or roads.

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Follow Dan Elliott at http://twitter.com/DanElliottAP.