President Vladimir Putin says that Moscow and Washington could jointly conduct criminal investigations into Russian intelligence officials accused of hacking during the 2016 U.S. election campaign.

Asked if Russia could extradite 12 Russian military intelligence officers indicted in the U.S. last week on charges of hacking into the Democratic election campaign, Putin challenged the U.S. to take advantage of a 1999 agreement envisaging mutual legal assistance.

He said the agreement would allow U.S. officials to request that Russian authorities interrogate the 12 suspects, adding that U.S. officials could request to be present in such interrogations.

Putin noted that Russia would expect the U.S. to return the favor and cooperate in the Russian probe against William Browder, a British investor charged of financial crimes in Russia. Browder was a driving force behind a U.S. law targeting Russian officials over human rights abuses.

Putin spoke after a summit with U.S. President Donald Trump in Helsinki on Monday.