The TSX was down 235.64 points to 14,312.07 at the close of trading.
A drop in the price of U.S. crude oil, which fell below $82 US before recovering to $82.09, down 72 cents on the day, also weighed on shares.
The Canadian dollar was trading at 88.96 US, down .09 of a cent on the day.
Losses picked up at mid-morning when reports began of shootings in Ottawa that killed a Canadian soldier and had Parliament Hill and area in lockdown.
"(The shooting) contributes to the negative sentiment," said Elvis Picardo, strategist at Global Securities in Vancouver. "The fact that the shooting happened in Ottawa may have rattled investors. Investors should be cautious about the ramifications."
Analysts said investors also may have been stepping back to reassess after four days of gains. The TSX fell more than 11 per cent in the previous two weeks amid declining oil prices and worries over global growth.
On Wednesday, the Bank of Canada nudged its growth estimate for Canada this year a point higher to 2.3 per cent.
It also dropped any reference to taking a neutral stance on interest rates, a move it signalled this month by saying it would not give forward guidance on rate moves.
Financials, the index's most heavily weighted sector, fell 1.2 per cent and the energy sector was off 2.2 per cent, a reflection of the dip in oil prices.
In the U.S., some strong earnings reports from Boeing and Yahoo initially helped lift stocks.
But they also finished lower as investors lost confidence in the four-day rally.
The Dow was down 153.49 at 16,461.32, the S&P 500 down 14.17 at 1,927.11 and the Nasdaq off 36.63 at 4,382.85.