While federal law allows former House speakers to maintain a government-financed office for up to five years to finish up matters relative to their tenure, they aren't permitted to use the office for private business or financial gain.
The taxpayers' tab for Hastert's office in Yorkville, Ill., about 50 miles from Chicago, is about $1.8 million.
The Tribune said it learned a secretary in Hastert's government office used email to coordinate some of his private business meetings and travel, as well as conduct research on one proposed venture.
A suburban Chicago businessman involved in the business ventures with Hastert said he met with the Illinois Republican at least three times in the government office to discuss projects, the investigation found.
Hastert told the Tribune he did not misuse the office.
"I didn't work on any private business out of there," he said.
Hastert, 70, spent more than two decades in Congress, leading the House for eight years until January 2007 and leaving later that year. He now lobbies for Dickstein Shapiro in Washington and has a consulting business, Hastert & Associates, in Illinois.
When told of the Tribune's findings, a government watchdog group called for an ethics investigation into Hastert's use of the office.