TORONTO - A bizarre crossbow attack in a Toronto library that left a father dead and his son charged with first-degree murder had police searching for a motive Friday.

The Main Street Library was full of school children, mothers and infants, and staff Thursday afternoon when Si Cheng, 52, was shot in the back with a crossbow.

Tearful witnesses to the terrifying spectacle were shepherded from the building by library staff. Neighbours saw a man who was carrying something in his hand get into a rental van and drive off.

Cheng was pronounced dead at the scene and his son, 24-year-old Zhou Fang, was arrested just minutes after the fatal incident.

Full-sized crossbows are not prohibited in Canada, nor do they require a licence or registration. Still, experts said it was more likely that Cheng was shot with a smaller crossbow that can be fired with one hand, which are illegal.

Fang, of Ottawa, was barely audible in court when asked by the judge if he understood the proceedings. Handcuffed and wearing a standard-issue prisoner's orange jumpsuit, he was remanded in custody on a first-degree murder charge and will appear in court again next Wednesday.

Earlier Friday, police would only say the two men knew each other, but Const. Tony Vella later confirmed they are father and son.

Media reports cited court documents showing Cheng had twice been convicted of domestic assaults, including one against Fang's mother in 2002 that Fang witnessed.

The use of a crossbow is "definitely a unique case in Toronto," Vella said.

"You hear about guns, you hear about knives being used, but you definitely don't hear about a crossbow."

Police are still trying to determine a motive, he added.

The killing was witnessed by "a lot" of people in the library. However, Vella said they are still looking for other potential witnesses to come forward.

"Some counselling is being offered to the witnesses because it was obviously a traumatic experience for everybody involved," he said.

Toronto Public Library spokeswoman Anne Marie Aikins said the popular neighbourhood branch was busy with its after-school and mother-and-children programs at the time. Babies were among those in the branch when Cheng was killed.

The fewer than 10 staff members in the library got everyone "out of the building quickly so no one else beyond the victim was hurt," said Aikins. The evacuation was orderly as possible but "there were upset people leaving, tears and so forth," she added.

Security varies at the library's 99 branches depending on their size and history of incidents, which have included assaults, disorderly conduct and suicides, said Aikins, who would not say whether the Main Street branch was equipped with surveillance cameras.

There have been no previous incidents at the branch, which remained cordoned off Friday for the investigation, she said.

There will be a review of security at the Main Street Library and a security guard will be hired when it reopens for patrons' "peace of mind."

Until then, library staff are off with pay. The branch is also contacting those who were in the library at the time, many of whom are regulars, to offer them counselling, she added.

A post-mortem was to be done on the victim either Friday or Saturday.

The east-end neighbourhood was abuzz Friday with news of the attack.

Residents discussed it in coffee shops, in restaurants and on street corners. Students at the nearby Kimberley Junior Public School speculated about what happened as they ate lunch in front of the school.

"It's all they're talking about this morning," said James Feistner, a parent who lives "right next door" to the library.

He called the news "surreal."

Lori Clyke was at the library just hours before the shooting. Clyke, who returned Friday after she heard the news, said she was relieved police called it a targeted attack.

"It's a bit different from someone walking in and randomly killing someone," she said.

If it was random, "this library will become off my list," Clyke added.

Vella would not reveal what type of crossbow was used in Thursday's killing but said the suspect had not been charged with any weapons charges. However, he said that didn't mean the weapon wasn't prohibited.

The RCMP, on its website, says the only crossbows prohibited in Canada are ones that can be fired with one hand and are 50 cm or less in length.

A licence or registration certificate is not required to own crossbows longer than 50 cm that require two hands to use. Such crossbows are sold in hunting shops across Canada. They can range in price from a couple of hundred dollars to $1,000 and more.

John Kerr, editor-in-chief of Ontario Out of Doors magazine, said an unrestricted crossbow would be an "awful big thing to be carrying around."

"It's not something you can hide under your coat," said Kerr, who suspects a smaller, prohibited crossbow was used to kill Cheng.

Cocking a full-sized crossbow is also a more involved affair. It requires a ratchet or string draw and there are few people strong enough to do it by hand, Kerr said.

"I've never heard of anything like this happening. It's very bizarre."

Incidents involving crossbows, while rare, aren't unheard of in Canada.

A Mission, B.C., father was charged in July with attacking his son with a crossbow. RCMP said the 36-year-old man was shot in the forearm with an arrow from the crossbow.

And in December 2009, Montreal police arrested a 42-year-old man following a six-hour standoff. Police later said the man had been holed up inside a house, armed with a crossbow.