NEEDHAM, Mass. —

Following a deadly shooting at a Minneapolis Catholic school, the former police commissioner who led Boston through the marathon bombing explained how investigators in Minnesota will work to "determine second-by-second what happened in an effort to try to identify why this person would commit such a heinous act."

A shooter opened fire Wednesday morning during Mass at the Annunciation Catholic School, killing two children and injuring 17 other people before killing himself, officials said. Latest Forecast

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Minneapolis Police Chief Brian O’Hara said the shooter — armed with a rifle, shotgun and pistol — approached the side of the church and shot through the windows toward the children sitting in the pews.

"After the

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