BOULDER, Colo. -- Nuclear Regulatory Commission inspectors have returned to a federal laboratory in Boulder after a June 9 plutonium spill.

The NRC said Wednesday the team has been expanded and will review lab conditions and the steps taken after the spill. The NRC inquiry began June 11.

About one-fourth of a gram of powder containing plutonium spilled at the National Institute of Standards and Technology when a vial cracked. Institute officials have said a small number of employees had internal plutonium exposure.

On Friday, NIST officials announced that a "small number of personnel" were shown to have internal plutonium exposure but they didn't say how many. Officials said internal plutonium exposure can lead to cancer and that the individuals affected were undergoing treatment to remove the plutonium from their bodies.

"We are concerned for the health and safety of our personnel and deeply regret these results showing internal plutonium exposure," said James Turner, NIST deputy director.

Officials say internal plutonium exposure can lead to cancer.

Officials have said radiation was found in two buildings.

The NRC says it has not identified any threats to public health or the environment.