A federal investigation into corruption in Suffolk County, N.Y., law enforcement has expanded, as the Justice Department examines at least a half-dozen cases to determine whether political considerations influenced decisions made by the Suffolk district attorney’s office, according to two people in law enforcement and three who have been questioned by the Justice Department.

Though few aspects of the investigation have become public, it has already upended politics in the Long Island county and led to public denunciations among erstwhile Democratic allies. Last month, Steve Bellone, the county executive, called for the district attorney, Thomas J. Spota, to step down, saying that he headed a “criminal enterprise.” Mr. Spota quickly lashed back at Mr. Bellone and said that there were “other reasons he wants me out of office,” suggesting that Mr. Bellone would prefer a less zealous prosecutor in office.

The half-dozen cases that are now part of the investigation, which began in 2013 and has already claimed a county police chief, were all handled by Mr. Spota’s top anticorruption prosecutor, Christopher McPartland. The highest-profile case being scrutinized involves Mr. McPartland’s investigation of a former county executive. Other cases include Mr. McPartland’s decision to order surveillance of the county press secretary and two lengthy wiretap investigations, which involved eavesdropping on a detective and a politically connected lawyer.

The New York Times identified the cases through interviews with two law enforcement officials and three people who have been questioned by the Justice Department or are aware of subpoenas. The five people spoke on the condition of anonymity either because they were not authorized to speak publicly about an active inquiry or because they feared the district attorney’s office would retaliate against them.