Canadian Liberal Prime Minister Justin Trudeau was hit by a handful of gravel, television images show.
He denies the allegations.
There's anger in Canada.
At least 233 people died in the West coast province between Friday and Monday.
According to a witness, the alleged attacker was wearing what looked like a military-style helmet and bullet-proof vest.
Four members of the family are dead and a nine-year-old is hospitalised with serious injuries.
The discovery of the remains at a former residential school in Canada has reopened wounds for survivors of the system.
Some of the children were as young as three years old.
PM Trudeau said the news "is a painful reminder of that dark and shameful chapter of our country's history".