* Ottawa confirms Canadian took part in attack that killed five
* Spy agency says 50-60 extremists have sought to leave Canada
* Says Canada not alone with homegrown radicalization
By Randall Palmer and David Ljunggren
OTTAWA, Feb 5 (Reuters) - The revelation on Tuesday that a Canadian-Lebanese dual national took part in the 2012 bombing of a tourist bus in Bulgaria underscores security agencies' concerns about the danger posed by Canadians joining attacks abroad.
It is not a problem that is exclusive to Canada. But last April the government's Canadian Security Intelligence Service said as many as 60 Canadians had traveled - or tried to travel - to Somalia, Afghanistan, Pakistan and Yemen to join al Qaeda-affiliated groups and engage in terror-related activities.
"I think this particular event demonstrates what they mean when they say that's of a serious security concern," Ray Boisvert, a former CSIS assistant director of intelligence, who retired last year after three decades at the agency, told Reuters.
"We have this ongoing problem that some persons living in Canada are linked to a number of issues, whether it's an old homeland issue like Sikh extremism, or it's still supporting the Real IRA, for example, or engaged in supporting Hezbollah or Sunni Islamic extremist groups. There is still fundraising for Hezbollah going on in Canada."
Bulgaria accused Hezbollah, the Lebanese militant movement, of carrying out last July's attack that killed five Israeli tourists. It said one of the three attackers had a Canadian passport and another had a Australian passport.
"I can confirm the individual in question is a dual national who resides in Lebanon," Canadian Foreign Minister John Baird told reporters.
He said the information on this person was much more robust than that provided by Algiers in regard to an attack in January on a natural gas plant in Algeria where about 70 people died.
Algerian Prime Minister Abdelmalek Sellal said a Canadian named only as Chedad coordinated the attack by a group of Islamic militants from a local al Qaeda group.
U.S. intelligence officials said they were concerned by signs that Canadian citizens were involved.
Canadian resident Ahmed Ressam, an Algerian citizen, tried to cross into the United States on a mission to blow up Los Angeles airport in 2000 and is serving 37 years in a U.S. prison.
Another Canadian, Ahmed Said Khadr, was a close associate of Osama Bin Laden and died in a clash with Pakistani forces, while his son Omar pleaded guilty to killing a U.S. soldier in Afghanistan and conspiring with al Qaeda.
Rumors that the hijackers in the Sept. 11, 2001, attacks on the United States reached the United States from Canada swirled for a long time afterward, but turned out to be wrong.
Boisvert said the United States had problems of its own. "There are lots of homegrown radicalization cases in the United States as well," he said, pointing to large numbers of Somali men returning from the United States to Somalia to fight.
Baird played down comparisons between the Bulgarian attack and the Algerian allegation about last month's gas plant attack, saying Canadian officials had been working constructively with their Bulgarian counterparts.
"We've had a more robust engagement with Bulgaria and they've provided more information. The situation in Algeria is completely different - we don't even have a name, which is obviously of concern," he said.
Baird said the Bulgarians were taking the lead in the case, adding he could say little about the man in question.
"It's not like they're a resident of Canada where we could have a substantial part of the investigation. They don't live or reside here," he told reporters, saying he was not aware of the last time the man had lived in Canada.
Lebanese immigration to Canada peaked during the 1975-1990 civil war. Canada's 2006 census, the last from which ethnic breakdowns were readily available, said there were around 170,000 Canadians of Lebanese descent living in Canada.
In addition, tens of thousands of Lebanese-Canadians live in Lebanon. (Editing by Janet Guttsman and Eric Walsh)