A small-scale nuclear reactor at a government neutron research facility was shut down last week after it began to spew elevated amounts of ionizing radiation. No employees received dangerous doses of radiation, according to officials, who are now devising a plan to determine the cause of the leak.

The incident happened on February 3 at the NIST (National Institute of Standards and Technology) Center for Neutron Research in Gaithersburg, Maryland, according to a statement put out two days later. The reactor was shut down and placed in a safety mode, and it won’t return to service until the “cause of the elevated radiation levels is determined and corrected,” per the statement.