WASHINGTON — Eleven people, including nine Marines, were being treated for injuries on Tuesday after an envelope carrying a hazardous but unknown substance was opened on a Virginia base down the street from the Pentagon, the authorities said.

Hazardous materials teams responded to the base in Fort Myer, Va., after two Marines who initially handled the envelope started having an unspecified physical reaction to the substance, said a military official familiar with the episode who spoke on the condition of anonymity.

The official said the envelope was addressed to the Marines’ headquarters battalion on the base, not to an individual. A Marine in the unit’s administrative section opened the envelope and, because it looked suspicious, called over another Marine to inspect it, the official said.