The FBI has been investigating a longtime Exxon Mobil consultant over the contractor's alleged role in a hack-and-leak operation that targeted hundreds of the oil company’s biggest critics, according to three people familiar with the matter.

The operation involved mercenary hackers who successfully breached the email accounts of environmental activists and others, according to the sources.

The scheme allegedly began in late 2015, when U.S. authorities contend that the names of the hacking targets were compiled by the DCI Group, a public affairs and lobbying company working for Exxon at the time, one of the sources said. DCI provided the names to an Israeli private detective, who then outsourced the hacking, according to the source.