US immigration officials reviewed tens of millions of voter records in a sweeping hunt for noncitizen voters, CNN reported on Tuesday.

Internal documents released by the advocacy group Democracy Forward describe the operation. A June report said investigators had taken in 50.6 million records from a Justice Department dataset, plus 48.4 million more obtained by senior DHS aide Heather Honey.

Homeland Security Investigations called it a phased national initiative. Officials say 1,620 investigations are now open and 151 arrests have been made.

ICE also plans to pay Thomson Reuters $125 million for access to personal data for the fraud hunt, 404 Media reported.

Critics call the program a surveillance overreach targeting voters. The administration says it is simply enforcing election law.

The disclosures are fueling a fierce debate over voter privacy ahead of the midterms. More document releases are expected.

Democracy Forward obtained the records through public records requests. The group says the program sweeps up innocent voters.