The mass shooting in Maine last week, which killed 18 people, was the country’s deadliest of the year. It may have also been one of the most avoidable. More than five months earlier, the Army Reserve and a Maine sheriff’s department had been made aware of a reservist’s deteriorating mental health. Just six weeks before the killings, he had punched a friend and said he was going to carry out a shooting spree. Nicholas Bogel-Burroughs, a national reporter for The Times, explains why so many warnings failed to stop the shooting. Guest: Nicholas Bogel-Burroughs, a national correspondent for The New York Times.