BOULDER, Colo. -- Colorado Rep. Mark Udall wants a federal agency to explain how traces of radioactive plutonium ended up in a sink at a Boulder laboratory and says it may have gotten into the city's sewer system.

A glass vial cracked at the National Institute of Standards and Technology last week and spilled a powder containing plutonium.

Contamination was initially found on a desk, a notebook and a chair used by a researcher. The institute said safety experts found plutonium in the sink when they re-entered the lab this week.

The institute said the researcher who had been working with the plutonium had washed his hands during the incident.

NIST officials said 22 workers were exposed and decontaminated with soap and water, while an additional two workers later said they were exposed.

Federal officials said workers are undergoing medical tests. Radiation from plutonium is weak but hazardous if ingested or inhaled.

Udall said the plutonium may have leaked into the city sewer system. He says the lab should review its safety procedures.