A series of emails exchanged between U.S. diplomats in Islamabad and State Department officials in Washington about whether to challenge specific U.S. drone strikes in Pakistan is at the center of a criminal probe involving presumptive Democratic presidential nominee Hillary Clinton's handling of classified information, the Wall Street Journal reported Thursday.

The Journal, which cited congressional and law enforcement officials briefed on the FBI probe, said that the 2011 and 2012 emails were sent via the "low side" — a government slang for a computer system for unclassified matters — as part of a secret arrangement that provided the State Department more of a voice in whether a CIA drone strike went ahead.

Some of the emails were later forwarded by Clinton's aides to her personal email account, which routed them to a server she kept at her home in suburban New York when she was the secretary of state, the officials told the Journal.

Clinton is being investigated over how classified information got on her private email server, and a recent report by the State Department inspector general found that she had broken government rules by using a private email server without approval, undermining Clinton's earlier defenses.

Law enforcement and intelligence officials said that the State Department discussions about the covert CIA drone program should have been conducted over a more secure government computer system designed to handle classified information, the Journal reported.