In September 2001, shortly after the Sept. 11 terrorist attacks, a mysterious powder called anthrax began showing up in letters addressed to two senators and several prominent media figures.

Five people were killed and 17 sickened.

Two weeks ago, the day after the seventh anniversary of 9/11 and two months after the FBI’s primary suspect in the anthrax mailings committed suicide, three letters containing threats were mailed to the Westmont and Willowbrook police departments and the DuPage Children’s Museum. Two letters contained white powder.

Though the powder tested negative for any hazardous substances, U.S. Postal Inspection officials are being tight-lipped about the investigation. Several clues, however, point to a link between the letters and a prostitution sting conducted Sept. 4 by the Westmont and Willowbrook police departments and the DuPage Metropolitan Enforcement Group. That bust, at an undisclosed DuPage County hotel, resulted in 14 arrests.

The Progress published a story about the prostitution sting on its Web site Sept. 8 after

discovering that one of the men arrested was a member of the DuPage Children’s Museum Board of Directors. Steven M. Gillhouse of Naperville, the board member charged with soliciting prostitution, confirmed his arrest to the Progress and said in a phone interview before the story was published that he planned to plead not guilty. Later that evening, he resigned from the board.

In subsequent days, the story was picked up by other Chicago media outlets and the Associated Press, and ran in newspapers across the country.

Four days after the Progress story, on Sept. 12, the Willowbrook Police Department received a letter in the mail containing threats and white powder. Willowbrook Deputy Police Chief Mark Shelton said the threats in the letter made mention of the prostitution sting.

“Yes, there was reference to (the sting, police involvement and the arrest of the former board member),” Shelton said. “But everything has been turned over to the postal inspectors at this point,” he added, declining further comment.

Next in line came the Westmont Police Department.

Westmont Sgt. Jim Gunther said after receiving the threatening letter, Willowbrook police called Westmont Deputy Police Chief Tom Mulhearn to ask if his department had received a letter. Gunther said Mulhearn checked the mail and found a letter matching the description addressed to a Westmont officer whose name was not disclosed. It was placed in a plastic bag as evidence, and Mulhearn took it to the Willowbrook Police Department where an FBI team was on its way to test the powder. The Westmont letter contained threats but no powder, and the Willowbrook powder tested negative for any hazardous material.

The following day, the DuPage Children’s Museum reopened after being closed for a day, and a receptionist opened a letter addressed to the museum containing white powder, said museum Executive Director Susan Broad. The museum called the Naperville Police Department, which tested the substance and deemed it nonhazardous.

The DuPage County Metropolitan Enforcement Group, the third law enforcement entity involved in the prostitution sting, did not receive a threatening letter, said the agency’s director, Jim Kuchta.

“We were surprised,” he added.

Westmont police and the museum declined comment on whether threats in their respective letters pertained to the prostitution sting or Gillhouse’s resignation.

Gillhouse did not return multiple phone messages left at his home. His court date for the misdemeanor solicitation charge is set for 9 a.m. Wednesday in DuPage County Circuit Court.

The Postal Service inspector’s surprise announcement of its investigation Sept. 23 has caused some confusion and left some questions unanswered.

In a news release, the inspector’s office said it was offering a $50,000 reward for information pertaining to “several letters containing white powder and threats” sent to the police departments and the museum.

When the Progress contacted Westmont Deputy Police Chief Tom Mulhearn that same day, he said he had no knowledge of the postal inspector’s investigation or its news release. When asked why the public was not informed earlier of the threats, Mulhearn said the Westmont and Willowbrook police departments had turned the investigation over to the FBI.

“We thought the FBI was handling it,” he said.

However, FBI spokesman Ross Rice said after the FBI tested the powder at a state-testing facility and deemed it nonhazardous, it was no longer a FBI concern.

“We didn’t turn the investigation over to the Postal Service because we never did an official investigation,” Rice said. “We provided assistance to local police departments.”

Though the powder was not hazardous and it posed no danger to the public, Postal Inspector Phillip Steele said the inspection office is aggressively pursuing the investigation.

“Any time there’s white powder in an envelope, we take it very seriously,” he said. “The intent is usually fear. We investigate these cases aggressively because of the fear they instill.”

However, Steele would not comment as to why it took 11 days for the Postal Inspector’s office to issue the news release. He also said it would not be appropriate to comment on whether any suspects had been identified, since the investigation is ongoing.

“We are working around the clock to wrap this up as quickly as possible,” Steele said.