(CNN) -- The National Security Agency's internal watchdog detailed a dozen instances in the past decade in which its employees intentionally misused the agency's surveillance power, in some cases to snoop on their love interests.

A letter from the NSA's inspector general responding to a request by Sen. Chuck Grassley, R-Iowa, lists the dozen incidents where the NSA's foreign intelligence collection systems were abused. The letter also says there are two additional incidents now under investigation and another allegation pending that may require an investigation.

At least six of the incidents were referred to the Justice Department for possible prosecution or additional action; none appear to have resulted in charges. The letter doesn't identify the employees.

Several of the cases involve so-called "Loveint" violations.

In one case, detailed by the NSA's watchdog, a civilian intelligence employee assigned overseas was found to have used the NSA's signals intelligence collection system to listen to the phone conversations on nine phone numbers belonging to foreign women from 1998 to 2003 without any valid reason. The signals intelligence system is used to spy on foreign targets for national security reasons.

The case began because a woman, a foreign national employed by the U.S. government, told another employee she suspected the man with whom she was in a sexual relationship was listening to her calls. The employee who misused the NSA's systems also incidentally collected the communications of a U.S. resident on two occasions, a move that requires a court warrant.

The NSA's vast surveillance powers are under fire after the disclosure of internal documents by former NSA contractor Edward Snowden. Intelligence officials have sought to defend the NSA's surveillance activities by saying the agency doesn't misuse its authority.

Grassley wrote to the NSA last month seeking to find out how often the NSA's authorities are misused.

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

In many cases the employees who intentionally abused the NSA's spying systems resigned before they could be punished. Several were demoted in rank or otherwise sanctioned.

In one 2004 case, a civilian employee told NSA security that she had spied on a foreign phone number because she found it on her husband's cell phone and suspected he was being unfaithful. She collected his phone calls. The employee's infraction was referred to the Justice Department, but she resigned before she could be fired.

Some of the violations appear to have been uncovered by the NSA's own auditing systems.Others were self-reported, including one during a polygraph of an employee.

One employee violated NSA's rules on the first day he had access to the agency's signals intelligence collection system.

He queried six e-mail addresses belonging to a former girlfriend. He told investigators he wanted "to practice on the system" using his former girlfriend's e-mail addresses and that he received no information, and hadn't read any emails. He was demoted and his pay was reduced, and the NSA's inspector general recommended he not be given a security clearance.