NEW YORK (AP) - Americans returning to work faced anxious moments opening envelopes after the number of people across the nation exposed to anthrax sent through the mail jumped.

Three new cases - a police officer and two lab technicians involved in an investigation at NBC's New York headquarters - tested positive for exposure to the bacteria, Mayor Rudolph Giuliani said Sunday. Nevada officials said four people who may have come into contact with a contaminated letter at a Microsoft office tested negative, while results weren't known for two others.

The new New York cases brought to 12 the number of people around the nation who either have anthrax or been exposed to it. That does not include an NBC employee who is taking antibiotics after displaying possible symptoms of the disease.

The anthrax scare began in Florida Oct. 4 when it was confirmed that an editor at the tabloid the Sun had contracted the inhaled form of the bacteria. He later died, the first such death in the United States since 1976.

Seven other employees of American Media Inc. have tested positive for exposure and are being treated with antibiotics. None have developed the disease. A second round of blood tests for more than 300 of the company's employees is expected this week.

In Washington, Attorney General John Ashcroft said it was premature "to decide whether there is a direct link" to Osama bin Laden's terrorist network, but "we should consider this potential that it is linked."

Some of the Sept. 11 hijackers spent time in Florida, and the FBI said the wife of the Sun's editor, who works as a real estate agent, rented apartments to two of them.

"Right. That's true," FBI spokeswoman Judy Orihuela said early Monday when asked of reports that Gloria Irish rented apartments in Delray Beach, Florida, to Hamza Alghamdi and Marwan al-Shehhi this summer. Her husband is Sun editor Michael Irish.

"Right now we consider it a coincidence," said Orihuela, spokeswoman for the FBI's Miami regional office, "because we don't have any tie between the anthrax and the terrorists."

News of the exposures has caused jitters around the world, with a number of false or pending cases reported over the weekend.

In New York, Giuliani said the officer and two technicians were exposed while working on the anthrax case involving Erin O'Connor, 38, the assistant to NBC News anchor Tom Brokaw. O'Connor is expected to recover.

"There's a very strict lab protocol in place, but what may have happened here, it's possible they may not have fit their masks properly around their noses and mouths," said Sandra Mullin, a health department spokeswoman.

O'Connor was exposed when she opened a letter containing a brown granular substance, which was mailed to Brokaw from Trenton, New Jersey. It was postmarked Sept. 18, one week after terrorist attacks on the World Trade Center and Pentagon.

Federal officials in New Jersey interviewed postal workers and watched surveillance videotapes Sunday as part of their efforts to trace the letter. Postal Service officials believed it could have been mailed from one of 46 post offices, FBI Special Agent Sandra Carroll said.

"It's very complex and a very comprehensive investigation that's a lot like looking for a needle in a haystack," Carroll said Sunday.

At first, O'Connor thought she had thrown away the letter, but remembered it Friday while being interviewed, New York postal inspector Peter Nash said Sunday.

Investigators had initially focused on a second letter - postmarked St. Petersburg, Florida - as the likely source of the anthrax.

In Nevada, where a letter sent to a Microsoft office in Reno contained pornographic pictures contaminated with anthrax, officials said tests of four people who may have been exposed proved negative for the bacteria.

Results for two others were pending, with results of those nasal swab tests likely known Monday, said Washoe County health officer Barbara Hunt.

The six people tested included five Microsoft workers and a family member.

"We're obviously relieved to hear that four of the employees who were tested, their tests came back negative," Microsoft spokesman Matt Pilla said. "It sounds like the other two may come back negative as well. That's obviously great news."

State officials have sent the pornographic pictures to the Centers for Disease Control and Prevention in Atlanta for further testing.

Health and Human Services Secretary Tommy Thompson said he would ask Congress for more than $1.5 billion to take steps to counter bioterrorism threats. Some of the money would be used to purchase enough antibiotics to treat 12 million people for 60 days, six times the current supply.