The only passenger rail link connecting Britain to the rest of Europe remained shut Sunday for a second day as engineers investigate the mystery behind a glitch that left thousands stranded for hours in the darkened tunnel running under the English Channel.

With Christmas only days away and more than 55,000 passengers' trips already canceled, the company is under intense pressure to find out what happened Friday, when five trains became stuck inside the tunnel overnight, leading to anger and panic among trapped passengers.

All services are suspended "until we get to the bottom of what happened Friday night," Eurostar chief executive Richard Brown told BBC television. "We will not start services again until we are sure we can get them through safely."

The exact cause of the electrical fault that disabled the five trains — as well as one more that broke down outside of the tunnel and others that malfunctioned Saturday — is unknown.

On Saturday, executives blamed the faults on the quick transition from the icy cold of France, which is suffering some of its worst winter weather in years, to the relative warmth of the tunnel, which could have produced condensation and interfered with the trains' electrical systems.

Transport commentator Christian Wolmar suggested that the trains may have fallen victim to snow buildup in their undercarriage, which might have fallen off all at once inside the tunnel, damaging their equipment. Writing in The Mail on Sunday, Wolmar also suggested that water from the melting snow on the trains' nose cones might have caused the malfunction.

"It's all a bit of a mystery and the company, and indeed a lot of people, appear baffled by it," said Nigel Harris, the managing editor of Rail magazine. "What is really puzzling about this is the fact that it is happening now, even though the trains have been exposed to cold weather over the last few years."

Comparable trains in France have "been going even longer than Eurostar without experiencing any of these cold-weather problems," he said. The cold weather has had little effect on France's TGV high speed trains, which have been running with little more than minor delays Sunday.

Eurostar has run test trains through the tunnel and is due to rule whether it is safe to resume travel shortly, the company said.

It added that the stoppage has already meant that about 31,000 people in Britain, France and Belgium have had to cancel trips Saturday, and 26,000 more were expected to be affected Sunday. With a huge backlog of passengers still building, Eurostar has blocked any sales until after Christmas and Brown warned that services may not be back to normal for days.

In the meanwhile, anxious travelers are waiting.

"I came again this morning at 5:30 and I am waiting to get on a Eurostar so I can get back to London," said 60-year-old Xavier Mallard, speaking from Gare du Nord, the terminus in Paris for Eurostar trains. "I am hopeful."

But for those seeking alternative routes between Paris, Brussels and London, the winter weather was dealing out more bad news.

Nearly half of all flights out of Paris' Charles de Gaulle and Orly airports were cut Sunday through mid-afternoon, with more cancellations forecast for Monday. Belgium was also badly hit, with passengers in Brussels lining up for hours in an effort to rebook flights.

Tourist Paul Dunn, 46, who was stuck in Paris, said he was looking for alternatives but that information was hard to come by.

"We said: 'Can we get the train to (the French city of) Calais and the ferry?' They are saying: 'We don't know what you can do. You can try."'

It is a measure of the popularity of the 15-year-old Eurostar service — which whisks passengers from London to Paris or Brussels in about two hours — that its closure has dominated news in Britain.

European parliamentarians on both sides of the Channel have criticized the train company as being irresponsible, while Britain's opposition Conservative Party said the issue was a matter of "huge concern."

Brown seemed to acknowledge that there were some problems but defended his staff.

"I'm not pretending it went well. I think it went quite a bit better than people say," he told the BBC.

The problems — and passengers' complaints about their treatment while trapped on board — could deal Eurostar "huge reputational damage," Harris said.

"They have promoted themselves as the 'green,' stress-free alternative to flying and now they face a major technical issue that they need to get on top of," he said.