Ukrainian national security service forces on Wednesday entered the headquarters of the country's natural gas company Naftogaz for criminal investigation, officials said.
"This is an investigative group, not people in masks ... This is all within the framework of the law," the security service's spokeswoman Maryna Ostapenko said.
"A standard investigation is now proceeding. Why make such a fuss?" she noted.
Valery Horoshkovsky, the first deputy head of Ukraine's national security service, told lawmakers in the parliament hall that it was a legal action.
Naftogaz spokesman Valentyn Zemlyansky said earlier that a group of unidentified armed men in camouflage broke into the company's building.
The customs service earlier said Naftogaz had legally taken over 11 billion cubic meters of gas owned by RosUkrEnergo -- a gas intermediary between Russia and Ukraine that had been criticized by Tymoshenko.
Naftogaz has been in the focus of disputes with Russia over payment for Russian gas supplies. The most serious dispute prompted a two-week cutoff of gas supplies in January that severely affected supplies throughout Europe.