MOSCOW Sun Feb 23, 2014 1:55am EST

(Reuters) – An explosion that investigators said may have been caused by gas killed three people at a shopping complex in Russia’s remote Altai region in southern Siberia on Sunday, law enforcement authorities said.

Russian authorities are on alert for any attacks during the 2014 Winter Olympics, which end on Sunday in the Black Sea resort city of Sochi. Initial reports gave no information about the cause of the blast, raising fears it could have been a bombing.

But the federal Investigative Committee made no mention of any suspicion of a militant attack or foul play in the early morning blast that killed three female employees at the complex in the city of Biisk, more than 2,500 km (1,500 miles) east of Sochi.

“According to preliminary information, a household gas explosion may have been the cause,” it said, adding that the investigation was continuing and a criminal case on suspicion of lethal negligence had been opened.

Suicide bombers killed 34 people in attacks in the southern city of Volgograd on December 29 and 30, increasing fears militants from the North Caucasus near Sochi would strike during the Games, a prestige project for President Vladimir Putin.

(Writing by Steve Gutterman; Editing by Alison Williams)