Rome ? A storm-tossed tree branch that hit Swiss power lines helped trigger a massive blackout in almost all of Italy on Sunday, trapping thousands on trains and forcing the pope to use a backup generator to proclaim his new cardinals. The outage, blamed for three deaths, underlined the dangers of Italy’s reliance on imported power.

It was Italy’s worst power outage since World War II. Most of the country’s 58 million people were affected — more than in North America’s biggest blackout, which left 50 million people without power in Canada and the United States on Aug. 14.

As in the North American blackout, there was initially confusion about the cause and finger-pointing among neighboring countries. With scant domestic supply and swelling public demand, Italy imports most of its electricity.

The blackout began at 3:25 a.m., hitting all of Italy except the island of Sardinia. The lights came back on in northern Italy by early morning and in most of Rome shortly after noon. Power was restored to the rest of Italy late Sunday, according to television news reports.

The blackout was blamed for the deaths of three elderly women, the ANSA news agency said.

Two women — one 81 and the other 72 — died in separate accidents when they fell down darkened stairs, the agency said. A 92-year-old woman died of burns after a candle set her clothes on fire.

As experts tried to work out the cause, none of the three countries involved wanted all the blame. Swiss and French energy officials said the responsibility was Italy’s, while the Italians noted that the power cut came from France.

Initial investigations indicated a chain reaction that started in Switzerland and moved through France.

In Switzerland, a tree branch hit and disabled a power transmission line. This caused another Swiss line to overload, which then knocked out French transmission to Italy.

Stranded people who were making their way home from an all-night festival wait for the train to arrive in Rome's Repubblica subway station, during a nationwide blackout. The cause of the blackout was still unknown late Sunday.

“After that, all connections to Italy dropped out,” said Rolf Schmid, spokesman for the Swiss power company Atel.

Authorities said there were no indications of foul play.

Italian energy company Enel agreed with the Swiss description of the chain reaction. But France’s electricity grid operator RTE said it was too early to speculate about possible causes.