CHICAGO (MarketWatch) -- More than 12 envelopes containing an unknown white and powdery substance have been sent to senior executives at The Wall Street Journal, and two floors of the newspaper's New York headquarters were evacuated, Dow Jones employees said Wednesday.

New York City police and hazardous-materials investigators are looking into the situation, a spokesman for the newspaper said earlier.

Three of the envelopes were opened, forcing officials to evacuate the 9th and 11th floors of the paper's headquarters and to temporarily quarantine five employees who came in contact with the substance.

The employees are "apparently in good spirits and health," according to an internal Dow Jones memo.

Some of the evacuated staffers were moved to other floors in the building, while others were relocated to Dow Jones offices in Jersey City and South Brunswick, N.J. The company's expectation is that employees will be able to return to the Manhattan headquarters on Thursday, the memo said.

Some of the envelopes are being isolated in the mailroom of the Journal's offices in Lower Manhattan, while others already have been distributed to other parts of the building, the spokesman added.

The business-size envelopes are marked with a Tennessee return address, according to the spokesman. No further details were available.

The Wall Street Journal is owned by News Corp. NWS NWS, -3.57% (NWS), which also owns other Dow Jones properties including Barron's, Dow Jones Newswires and MarketWatch, the publisher of this report.

Last October, an envelope containing a suspicious substance was received at the New York Times' headquarters in midtown Manhattan. After the lobby was closed for four hours, police concluded that the substance was not harmful.

A similar incident temporarily shut down the newsroom at Reuters' offices in New York that same month. In the Reuters case too, no hazard was discovered.

Since the Sept. 11, 2001 terrorist attacks, there have been a number of mailed threats that sparked fears of bioterrorism. In October of that year, an NBC News employee tested positive for anthrax after handling two envelopes containing a white powder and a sandlike material addressed to then-"NBC Nightly News" anchor Tom Brokaw. The envelopes also included letters threatening Brokaw.