"Based on the information available at this time, this incident was preventable," James M. Turner told the House Science and Technology's subcommittee on technology and innovation.
Turner said medical experts report that no significant health effects are likely for the people involved.
The Nuclear Regulatory Commission ordered the Boulder laboratory to stop using radioactive materials until it can show its procedures are safe, and that order remains in effect.
Subcommittee chairman David Wu, D-Ore., expressed his unhappiness at the June 9 incident, commenting that "along with scientific and technical excellence must be an equal dedication to safe laboratory practices."
The spill occurred when a test tube containing plutonium powder cracked and some of the powder spilled, according to testimony prepared for the hearing.
"I am very disappointed that we are all here today - this incident never should have occurred in the first place," said Rep. Mark Udall, D-Colo., who represents Boulder.
Turner said multiple investigations of the incident have been completed or are under way.
Trace contamination was found on some workers, he said, and was later found in the men's restroom and in other parts of the building.
On June 12, it was discovered that a worker had walked to other parts of the building before being decontaminated, and traces of radioactive material were found in an office on a desk, chair and lab notebook, Turner said in prepared testimony.
And, on June 14, a study of the lab where the accident occurred revealed contamination in the sink. The worker most involved in the incident was interviewed again and disclosed that the worker had used a sink for hand washing.
Turner said the City of Boulder was then informed that contamination may have been released into the wastewater system.
The probable cause of the incident was handler error, Turner said, adding that the agency needs to improve training and is focusing on its overall approach to the environment, health, safety and emergency response.