HONOLULU -- A criminal investigation is underway in which city street sweeper employees are accused of padding their paychecks with overtime and paying kickbacks to their boss, KITV reported.

Police have opened a theft and bribery case and have already arrested five people, including a supervisor. It started with anonymous tip.

The investigation centers on four city street sweepers and their supervisor who work out of a city base yard on Sand Island.

The workers are accused of getting paid for working extra weekend days when they never showed up for work, sources said.

Four employees falsified their time sheets and their supervisor approved the bogus overtime, in exchange for kickbacks from his underlings, sources told KITV.

About six weeks ago, sources said police executed a search warrant, seizing documents and payroll records from the Sand Island base yard offices.

Last month, police arrested supervisor Manuel Castro on suspicion of theft, bribery and tampering with a government record, and released him without charges, pending further investigation.

The other four employees were also arrested and questioned, but released.

The accused employees at first denied any wrongdoing, but once they were confronted with the evidence, which included the falsified payroll records, they admitted to the overtime scheme.

The men operated the overtime scam for more than one year, sources said. Officials are still trying to calculate how much money was involved.

The accused employees work in the city's Facilities Maintenance Department, which also handles pothole patching and repaving roads.

Its director, Jeoffrey Cudiamat, released a statement that said, "This matter was properly referred to the Honolulu Police Department immediately after information about it emerged.

City budget documents show nearly 30 percent of the positions in the department were vacant as of late January. So the city relies on overtime to fill in the gaps, and high overtime bills might not have set off any red flags.

The city is conducting an internal review of the case. In addition to pending criminal charges, the employees will probably face administrative punishment. The most serious of which would be termination.