WASHINGTON (Reuters) - Eleven people fell ill and three were transported to a hospital on Tuesday after a letter was opened at a U.S. military base in Arlington, Virginia, the fire department and a base spokeswoman said.

“Someone opened a letter and everyone at the office started feeling badly,” said Leah Rubalcaba, a spokeswoman for Joint Base Myer-Henderson Hall.

The three people taken to a hospital were in stable condition, Arlington Fire Department said on Twitter.

The letter was opened in the Marine Corps’ portion of the base, which is located near Washington.

“An envelope containing an unknown substance was received” at the base, the Marine Corps said in a statement.

The building was “screened and cleared” and the letter removed, it said.

The Naval Criminal Investigative Service and the Federal Bureau of Investigation are conducting a joint investigation.