An anonymous letter sent to an administrator at Riverside-Brookfield High School April 6 has prompted Riverside police to issue a statement emphasizing that the threat mentioned in the letter was not related to the high school but appears to be related to an unspecified incident at Brookfield Zoo in 2003.

Riverside Police Chief Thomas Weitzel said that his department is investigating the incident with Brookfield police and the Brookfield Zoo police.

“We’re going to see if we can gather evidence from the letter itself,” Weitzel said. “Although there’s no direct threat, we wanted to get the information out there that there’s no direct threat to the high school or any student at the high school.”

The letter reportedly referred “to an incident that occurred at the Chicago Zoological Society in 2003 and that a child may be kidnapped in Brookfield,” according to a press release issued by Weitzel last week.

David Bonnette, interim superintendent at Riverside-Brookfield High School, said that the letter was addressed to Assistant Principal Tim Scanlon and received in the regular mail delivery to the school on April 5. The letter wasn’t opened until the next morning, Bonnette said.

“There has been, over time, a whole series of these kinds of letters that came out,” Bonnette said. “There’s no particular event in this case we know of that would’ve precipitated this.”

While police did not elaborate on the connection to a “2003 incident at Brookfield Zoo,” the Landmark reported in November 2003 that Stuart Strahl, CEO of the zoo, received an envelope that contained a threatening letter and white powder from an unknown person.

That letter referred specifically referred to an incident in which a zoo police officer shot and killed a wolf that attacked a woman who had strayed too close to its exhibit space.

In March 2004, envelopes containing white powder and a threatening letter were sent to the Brookfield Fire Department, Brookfield-North Riverside Water Commission and Brookfield Zoo. A similar letter was also sent to the Lyons village hall.

The contents of the letter or letters received at that time were not disclosed. The letters from both dates were turned over to the FBI.

A spokeswoman for Brookfield Zoo declined to comment on the case.

The envelope sent to RB High School this morning did not contain a white powder, only a letter, Weitzel said, adding that Riverside police intended to send the letter to the Illinois State Police crime lab.