Snowden's documents showing that the NSA targeted Merkel's cellphone caused an uproar in Germany. That prompted the chancellor's governing coalition and opposition lawmakers in March to establish a parliamentary committee to investigate the scope of NSA spying in Germany.
The committee decided unanimously to invite Snowden to testify, the German news agency dpa reported. But lawmakers were unable to agree on whether Snowden should attend in person — as is usual for witnesses — or whether he could be questioned in Russia, where he has been granted temporary asylum.
Opposition parties insist Snowden should be brought to Berlin because he is a key witness. The government coalition is split on the issue, with Merkel's conservative bloc opposed to letting him into the country. The center-left Social Democrats, also members of the coalition, said all options remain open.
The final decision is likely to rest with Snowden himself, although German authorities could block him from entering since he doesn't have a valid U.S. passport.
The German government warned last week that inviting Snowden to appear before parliament in person could harm Germany's relations with the United States.
The head of Germany's domestic security service said Thursday that current cooperation with U.S. intelligence agencies was good. Hans-Georg Maassen, head of Germany's Federal Office for the Protection of the Constitution, told reporters that regardless of where Snowden is questioned, he would be unlikely to shed much light on the NSA's espionage activities in Germany.
"I can't imagine that he, in his function as a data administrator ... would be able to tell us very much about the content" of the files he leaked, Maassen said.
Among other witnesses who will be invited to testify before the inquiry are Merkel, her predecessor Gerhard Schroeder, current and former German foreign ministers as well as former NSA employee William Binney and former U.S. Air Force drone operator Brandon Bryant, dpa reported.