Former Pakistan prime minister Benazir Bhutto urged Canada on Tuesday to stay the course in Afghanistan, as a fresh threat on her life arrived the same day, underscoring her message about the need to rid her country of extremists.

"I can understand that Canada has suffered a lot of casualties, and I can understand that the imperatives that would be there for Canada to pull its team," Bhutto told CBC News Tuesday in Karachi. "But I do believe that the future of Pakistan and Afghanistan is critical to the direction the global community is going to take."

Bhutto acknowledged that Taliban militants have regrouped in Pakistan's tribal areas along the shared, porous border with Afghanistan. Many of those militants routinely cross over to launch deadly attacks on NATO troops, she said.

But if Canada and other NATO nations withdraw too soon, Bhutto said, she feared the Taliban could regain power and lead to reinvigorated Islamic extremism.

As she spoke about the NATO mission in Afghanistan and the implications it could have for the future of her country, Pakistan authorities were investigating a two-page, anonymous letter written in Urdu and delivered to Bhutto's lawyer, Senator Farooz Naik. The unidentified author of the note claimed to be sympathetic to al-Qaeda and Osama bin Laden, and threatened to kill Bhutto "by any means."

While its authenticity has not yet been verified, Naik said "we cannot take anything lightly," especially after Thursday's suicide attack targeting Bhutto during her homecoming from an eight-year, self-imposed exile.

Until last week, Bhutto had lived in Dubai since 1998 after being charged in Pakistan with corruption. She returned to Pakistan on Thursday after President Gen. Pervez Musharraf agreed to grant her amnesty and drop the corruption charges.

'More than likely' 2 suicide bombers

Bhutto and President Musharraf, longtime rivals, share the common goal of combating al-Qaeda and the Taliban. The two moderates have been negotiating on a power-sharing plan.

Blasts erupted Thursday as her caravan travelled through Karachi, where thousands of supporters showed up to greet her. Bhutto survived the attacks, but 136 others lost their lives.

It was believed initially that the attack was the work of a lone suicide bomber, but Sindh provincial Gov. Ishrat ul-Ebad Khan said on Tuesday "it was more than likely" that two suicide attackers were involved, after police found a second severed head.

No arrests have been made yet, Khan said, but police were questioning people connected to seven previous suicide attacks in Karachi, hoping to gain insight on Thursday's attack.

Political sniping has become heated in recent days, with Bhutto and the ruling party trading accusations about who orchestrated the bombing. Conspiracy theories often gain traction in Pakistan's intrigue-filled politics, and Chaudhry Shujjaat Hussain, chief of the ruling Pakistan Muslim League-Q party (PML), alleged in an apparent sarcastic swipe that Bhutto's husband planned the bombings to rouse public sympathy.

With files from the Associated Press