A hospital emergency room was locked down for more than two hours after a postal worker came in who had been exposed to a suspicious powder in a letter, officials said.

In New York City, three people prompted a similar response at a Brooklyn hospital after one received white powder in the mail.

Initial tests in Iowa indicated the substance found Thursday at the city's main post office was a mixture of rat poison and wheat flour, fire department spokesman Brian O'Keefe said. The worker, who had come in to Mercy Medical Center around 5 a.m., was checked out and released and the investigation was continuing.

"She walked in and said she was contaminated or something like that, so they locked it all down until they can figure out exactly what she got into," police Sgt. Misti Allison said.

In Brooklyn, three family members went to Wyckoff Heights Medical Center (search) on Thursday night after one of them, a 24-year-old woman, said she had received an envelope containing white powder in her mailbox earlier in the week.

She said she later developed flu-like symptoms, including a stuffy nose.

Hospital officials temporarily quarantined a part of the emergency room while the patients were examined. The woman, her husband and brother were all kept overnight and given antibiotics.

The powder was under analysis by the city health department, and there was no indication it was anthrax, officials said.

Meanwhile, in Des Moines, the letter was turned over to the Postal Inspection Service. Officials said it had no return address and declined to say whom it was addressed to.

"These situations since 9/11 more often than not turn out to be a joke or the substance turns out to be something that is not dangerous," hospital spokeswoman Daphne Christensen said. "Everything is just a precautionary measure."