A pipe bomb was delivered to a Chicago skyscraper a day after a similar device arrived at a Kansas City business, and federal agents were investigating possible connections.
Both devices were defused without incident.
The FBI said a threatening note accompanied the explosive device that arrived Wednesday at American Century Investments' midtown Kansas City mail facility, a few blocks from the company's national headquarters.
A similar explosive was found Thursday at a business in a downtown Chicago office tower, said Wanda Shipp, a Postal Inspection Service spokeswoman.
That package also contained a note, though it was not threatening, Shipp said.
Authorities did not name the Chicago business.
Investigators were searching for links between the two bombs, said Dan Taylor, a regional spokesman for postal inspectors in St. Louis.
In Kansas City, Postal Inspector Don Obritsch said his agency was working with the FBI and ATF on the American Century case.
"The guys are working on it feverishly," Obritsch said. "It's a big deal to us."
Obritsch said Wednesday's bomb appeared designed more to frighten than kill. "This was the real deal, but it was not primed to go off," he said.
Obritsch said investigators have not established a motive. The FBI declined to say whether the bomb was addressed to an individual or to American Century generally.