The world learned in early June about the National Security Agency's stunning capability to spy on just about anyone it wants to. Now we're finding out that power was just too tempting for some of its own employees -- with the agency acknowledging that workers used NSA tools to spy on love interests.

In a letter to Sen. Chuck Grassley, R-Iowa, NSA Inspector General George Ellard admitted that since 2003, there have been "12 substantiated instances of intentional misuse" of "surveillance authorities," and "SIGINT," or signals intelligence.

Just about all of these cases involve an NSA employee spying on a girlfriend, boyfriend or some kind of love interest, or "loveint." Media reports had earlier claimed NSA workers were engaged in this kind of activity. The letter to Grassley gave specific details for the first time.

According to the letter, just prior to a polygraph examination in 2011 one NSA employee admitted that he queried information on his girlfriend's phone "out of curiosity." However, that "subject retired in 2012 before disciplinary action had been taken."

Another employee went much further, tracking nine different telephone numbers for "female foreign nationals, without a valid foreign intelligence purpose" between 1998 and 2003 -- and listening to the phone conversations. The activity was uncovered after a female foreign national employed by the U.S. government, who was having sexual relations with the offending employee, told a colleague she thought her phone was being tapped.

In another instance, a female NSA employee admitted in 2004 to tapping a telephone number she found in her husband's cell phone "because she suspected that her husband had been unfaithful." In this case the NSA employee resigned before any disciplinary action.

The IG wrote that there are two additional open investigations into similar misuse of intelligence capabilities and yet another allegation for possible investigation.

Grassley, the top Republican on the Senate Judiciary Committee, said in a statement that the NSA should have a zero-tolerance policy toward abuse.

"I appreciate the transparency that the Inspector General has provided to the American people. We shouldn't tolerate even one instance of misuse of this program," he said. "Robust oversight of the program must be completed to ensure that both national security and the Constitution are protected."