HALIFAX - Four stowaways on a cargo ship managed to slip by guards at a Halifax port gate on Sunday and take a cab to Truro, N.S., before being captured, prompting the chair of the Senate's defence committee to question security at the harbour.

Three of the men were being held Monday in a Halifax jail while the other was under guard in a Truro hospital recovering from a respiratory illness.

The four were arrested at the train station in Truro on Sunday afternoon.

Laurie Gillmore, spokeswoman for the Canada Border Services Agency, said Monday it was "a good possibility'' the four men are Algerian, adding officers were still interviewing them to confirm their nationality.

She said the interviews and hearing would determine if the men pose a security risk or are a risk to flee.

Gillmore declined to provide details of how exactly the men got off the cargo vessel in Halifax and made it Truro, about an hour's drive away. The spokeswoman said the federal Privacy Act prevented her from revealing those details.

However, The Canadian Press has learned that the men were aboard an empty cargo bus that was transported aboard the Atlantic Container Lines vessel Atlantic Cartier from northern Europe to Halifax.

A source said the men managed to slip off the vessel, get to the security port's gate, then flee from guards after they were asked to produce identification.

Truro police spokesman Staff Sgt. Randy MacKenzie said the men attempted to board a train at the Via Rail station in Halifax.

When they were refused a ticket, MacKenzie said they took a taxi to the Truro station.

Truro police arrested them at the station after Via officials noticed the men attempting to use foreign currency to buy tickets to Montreal.

"Three of them were attempting to buy tickets with French money and that told the story right there,'' said MacKenzie.

Colin Kenny, chair of the Senate committee on defence and national security, said the incident shows the need for improved scanning of all cargo coming off container ships.

"This should trouble us greatly,'' said Kenny.

He noted that ports in the Far East have gamma-ray scanning systems that take images of all cargo coming off the ships, aiding in the detection of stowaways.

"We think that ports in Canada, and particularly a port as important as Halifax, need to move in that direction,'' said Kenny.

Gillmore said her agency is looking into the incident to see if improvements are needed.

"We will look into the circumstances and see if there's something that could have been done differently,'' she said.

Asked if the incident prompts wider concerns about security, Gillmore responded: "No, not at all. I wouldn't say so because it was taken care of swiftly.''

The four men are expected to be brought before officials with the Immigration and Refugee Board on Tuesday for a hearing into whether they will stay in detention.

Lee Cohen, a Halifax-area immigration lawyer, was concerned that the border security agency wasn't revealing more information on the case.

"It's mysterious,'' he said. "Usually the government will quickly confirm one way or another that someone is stowed away and come ashore in Halifax.''

Television stations reported on the stowaways Sunday, but the agency declined to confirm how the four men entered Canada when it issued a news release Monday morning.

Last April, speculation that the Cypriot-registered Cala Puebla was carrying up to 200 stowaways to Halifax turned out to be false.

However, the arrival of stowaways in the port of Halifax has been a problem in the past, with Canadian immigration officials saying in 2001 that up to 30 a year were arriving in the city.

In December 2002, 12 stowaways from Romania arrived aboard a container ship from Spain.

Companies face fines of up to $15,000 per stowaway if it's determined that sufficient security measures weren't taken to prevent them getting on a ship.