WASHINGTON — The Environmental Protection Agency on Tuesday barred reporters from three news organizations from an event on the impact of toxic chemicals on drinking water at the agency’s headquarters.

The event, during which the E.P.A. administrator Scott Pruitt declared that addressing the impact of a class of man-made chemicals was a “national priority,” came at a time when Mr. Pruitt is the subject of at least 12 federal investigations.

Among those denied entry from the morning session of the planned two-day event was a reporter from The Associated Press, Ellen Knickmeyer. When she requested to speak to an E.P.A. public affairs official, she was “grabbed by the shoulders and shoved out of the building by a security guard,” according to a report from the wire service.

Also turned away were Corbin Hiar, a reporter for E & E News, and Rene Marsh, of CNN, along with a camera operator and a producer from the cable network.