TRENTON -- Howard Birdsall, the former CEO of an influential engineering firm who admitted flouting state campaign finance laws to funnel illegal donations to politicians, was released from prison six months into his four-year sentence, NJ Advance Media has learned.

Birdsall pleaded guilty this year to second-degree misconduct by a corporate official after investigators found executives and employees at his now-defunct firm were reimbursed by the company for campaign donations made in their own names.

Company records obtained by The Star-Ledger in 2013 showed that hundreds of politicians, from mayors and freeholders to major state power brokers, received more than $1 million from the firm.

As those campaign contributions flowed out, Birdsall Services Group was receiving millions of dollars worth of public contracts.

Birdsall was given the four-year sentence on April 22, but was released from prison less than half a year into his term. His attorney, John McDonald, said Birdsall entered into the state's Intensive Supervision Program on October 20.

The program, known as ISP, "provides an opportunity for a carefully selected population of offenders, sentenced to state prison, to be released into the community," according to a fact sheet from the Administrative Office of the Courts.

It's the same program that allowed disgraced former Middlesex County sheriff Joseph Spicuzzo to avoid serving a full nine-year sentence for bribery.

The fact sheet describes ISP as a "rigorous form of community supervision" that requires participants undergo drug and alcohol monitoring, obey a 6 p.m. curfew and obtain employment.

Birdsall, now 73, is retired. A spokesman for the judiciary said community service requirements are sometimes increased for retired individuals who enter into the program.

Through his attorney, Birdsall declined an interview request.

Meanwhile, two other top officials from the firm who received lesser sentences for their roles in the scheme, including Birdsall's younger brother, remain incarcerated, records show.

Thomas Rospos, the former executive vice president, is serving a three-year sentence for a charge of tampering with public records. He is being held at the state prison system's Central Reception and Assignment Facility in Mercer County with a parole eligibility date of March 9, 2017, prison records show.

William Birdsall was given 270 days in county jail as a condition of a two-year term of probation after pleading guilty to misconduct by a corporate official. He is being held at the Ocean County jail with a release date of Feb. 20, according to county corrections officials.

Another executive facing jail time, chief administrative officer Scott McFadden, is still awaiting sentencing. He pleaded guilty to corporate misconduct in January.

S.P. Sullivan may be reached at ssullivan@njadvancemedia.com. Follow him on Twitter. Find NJ.com on Facebook.