Police say white powder sent in envelopes to the Wall Street Journal building in New York was harmless and probably was flour.
Authorities say similar mail was received Wednesday at Harvard Law School in Massachusetts.
No injuries were reported.
About 250 employees were evacuated. Five employees were being decontaminated as a precaution, said FBI spokesman James Margolin.
At least four of the envelopes were opened; others were left unopened in the mailroom, police said.
A newspaper spokeswoman said the mail was addressed to several executives. The postmark was Knoxville, Tenn., but each letter had a different return address.
The envelopes were found in three locations within the building in Lower Manhattan, said Margolin.
The 11th floor is shared by newspaper executives and editorial page employees. The 9th floor houses the newsroom and mailroom.
Employees who weren't needed immediately to publish the Wall Street Journal, or Dow Jones' other publications, were sent home. Other staff members were working on two unaffected floors and at the nearby Marriott World Financial Center.
The Department of Homeland Security was monitoring the situation but said other agencies were taking the lead.
The Wall Street Journal is owned by News Corp., the parent company of FOX News.