An unknown white powder was found in about a dozen envelopes mailed to the Wall Street Journal in New York City forcing the evacuation of about 250 employees from the paper's Manhattan newsroom, according to Dow Jones.
Similar mail turned up at Harvard Law School in Massachusetts, authorities said.
Five Journal employees were decontaminated as a precaution, said FBI spokesman James Margolin.
At least four envelopes were opened and others were left unopened in the newspaper's mailroom, police said.
Field tests are being done on the substance. People who came into contact with the substance were isolated.
The paper's Web site says the mail was addressed to several New York-based executives of the paper, which is owned by News Corp. The postmark was Knoxville, Tenn., but each letter had a different return address.
It said at least 10 of the envelopes were being held in the mailroom of the Journal's Lower Manhattan headquarters. The evacuated floor is shared by newspaper executives and editorial page employees.
The Department of Homeland Security was monitoring the situation but said other agencies were taking the lead.