BERLIN - German prosecutors have opened an investigation into the alleged monitoring of Chancellor Angela Merkel's cellphone by the US National Security Agency, officials said Wednesday, in a move that could again complicate diplomatic relations between the two allies.

It wasn't immediately clear what the new investigation might mean in terms of possible prosecutions of Americans.

Documents provided by National Security Agency leaker Edward Snowden indicated in October the US was monitoring Merkel's cellphone conversations, as well as those of 35 other foreign leaders. Merkel expressed outrage and accused Washington of a grave breach of trust.

In the ensuing diplomatic fallout, President Barack Obama acknowledged Germany's anger and promised that new guidelines would cut back on such monitoring, except in the case of a national security interest.

"The leaders of our close friends and allies deserve to know that if I want to learn what they think about an issue, I will pick up the phone and call them rather than turning to surveillance," Obama said at the time.

Following the news of the German probe, Obama's deputy national security adviser, Ben Rhodes, said the US believes direct dialogue between the two countries rather than an investigation is the best way to address Germany's concerns.

"We believe we have an open line and good communication" with Merkel and her team, Rhodes told reporters aboard Air Force One as Obama flew to Brussels for a meeting of the Group of Seven nations.

After mulling for months whether to open a formal probe, Chief Federal Prosecutor Harald Range determined "that sufficient factual evidence exists that unknown members of US intelligence services spied on the mobile phone of Chancellor Angela Merkel," his office said.

"The operation of a foreign intelligence service's secret agents is a criminal offense in Germany," Range told reporters after the decision was announced. "And that's not dependent on whether it is the intelligence agency of a friend or of another nation."

Range refused to say how he would interrogate witnesses, saying he couldn't comment on any specifics about the investigation.

In a similarly thorny diplomatic case, Germany got as far as issuing warrants for 13 unidentified CIA agents suspected of kidnapping a German terrorism suspect and taking him to a detention center in Afghanistan. The case was shelved in 2007 after the US Justice Department said extraditing the agents would harm "American national interests."

In his Wednesday announcement, Range's office said he was not opening a formal investigation of wider allegations of blanket surveillance of telecommunications data in Germany by US and British intelligence, saying that there was not yet sufficient factual evidence of concrete crimes. His office said that will remain under consideration.

Merkel's spokesman, Steffen Seibert, declined to comment on Range's decision or on whether the government fears it will weigh on relations with the US

The government didn't exert any influence on the prosecutor, Seibert told reporters. "I am not going to evaluate here the decision he has made," he said.

Separately, the German Parliament earlier this year set up a committee to investigate the scope of spying by the US and its allies in the "Five Eyes" network - which also includes Britain, Canada, Australia and New Zealand - in Germany.

It is seeking testimony from Snowden. Opposition parties want him brought to Berlin to testify, but the government argues that that would hurt trans-Atlantic relations and security cooperation with the US Snowden is currently in Russia.