Washington, June 11 (RHC)-- Hillary Clinton, the presumptive Democratic presidential candidate, is being investigated for using a private e-mail server as secretary of state.

E-mails between U.S. diplomats in Islamabad and State Department officials in Washington about whether to challenge specific U.S. drone strikes in Pakistan are at the center of a criminal probe involving Hillary Clinton's handling of classified information, the Wall Street Journal reports.

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

Some of the e-mails were then 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 secretary of state, the officials said, according to the newspaper.

Investigators have raised concerns that Clinton's personal server was less secure than State Department systems, and a recent report by the State Department inspector general found that Clinton had broken government rules by using a private email server without approval, undermining Clinton's earlier defenses of her emails.

The still-secret e-mails are a key part of the FBI investigation that has long dogged Clinton's presidential campaign, the officials told the Journal.

Clinton this week clinched the Democratic presidential nomination for the November 8th election and was endorsed by President Barack Obama.