BOULDER, Colo. (AP) - Nuclear Regulatory Commission officials have ordered a federal laboratory to stop using radioactive materials until it can show its procedures are safe after a plutonium spill.

The Boulder Daily Camera reports that the commission said Wednesday such stop orders are rare.

A vial cracked June 9 at the National Institute of Standards and Technology lab. About one-fourth of a gram of powder containing plutonium spilled.

Institute officials have said a few employees had internal plutonium exposure and were being treated.

Officials say radiation was found in two buildings and that internal plutonium exposure can lead to cancer.

Commission officials say no threats to public health or the environment have been identified.

James Turner, NIST deputy director, has called the spill "unacceptable" and said in a statement Wednesday they are investigating how it happened and how to prevent spills.

"We have fully cooperated with Nuclear Regulatory Commission inspectors and will continue to do so," he said.

Turner said NIST is consulting with medical experts about the health effects of plutonium exposure and making sure the affected personnel receive treatment.

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Information from: Daily Camera, http://www.thedailycamera.com