DENVER (AP) - A cab company offered to adjust the lease rate that its drivers pay for their cabs in exchange for phone calls to lawmakers opposing a bill, an allegation that could violate state lobbying laws.

The secretary of state's office on Tuesday said it will further investigate a complaint against Colorado Cab Co., LLC, which operates Yellow Cabs in Denver and Boulder. Legal watchdog group Colorado Citizens for Ethics in Government filed the complaint in March.

According to the group, drivers for Yellow Cab were offered adjustments on their lease rate worth $50 to $110 for calling legislators in opposition to House Bill 1114, which relaxed regulations governing the taxi industry. The group said drivers were given a list of lawmakers to call and a script.

In a statement, the secretary of state's office said it will investigate whether the company violated laws concerning the employment of unregistered lobbyists and whether drivers broke the law by failing to register and report as professional lobbyists.

An after business hours message left at Colorado Cab was not immediately returned, but the secretary of state's office said during its investigation the company said it: "has never paid money to, or given consideration to, a cab driver in exchange for lobbying."

State law requires that lobbyists register and it prohibits anyone to pay an unregistered lobbyist to engage in lobbying. Each violation of the law is a misdemeanor punishable by a fine of up to $5,000, and/or a year in jail.