KIEV, March 16 - Ukraine’s SBU security service obtained documents from the government concerning the disputed purchase by Naftogaz Ukrayiny of natural gas once owned by gas trader RosUkrEnergo, SBU chief Valentyn Nalyvaychenko said Monday.

The development is part of the investigation that has attracted attention earlier this month when SBU’s special forces, Alfa, had broken into the offices of Naftogaz, sparking critical remarks from Russian Prime Minister Vladimir Putin.