White powder in about a dozen envelopes sent to Wall Street Journal and Dow Jones executives turned out to be harmless, the New York Police Department said on Wednesday.

The letters contained flour or another food-based substance, police spokesman Paul Browne told Reuters. He said that such letters often are hoaxes.

The envelopes went to Journal Managing Editor Robert Thomson, Dow Jones & Co Chief Executive Les Hinton and Journal Editorial Page Editor Paul Gigot, a source familiar with the matter told Reuters.

Dow Jones is a unit of Rupert Murdoch's international media conglomerate News Corp, and owns the Journal.

About 10 more envelopes turned up in the Journal's mailroom, spokesman Robert Christie said earlier in the day. The envelopes were sent from Knoxville, Tennessee.