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It was late Thursday afternoon when the police in a suburban New Jersey town got a tip from a police department in Delaware: A man might be headed to a school there and could be armed.

Within minutes, police officers in Westfield, N.J., about an hour west of New York City, arrived at the building, Tamaques Elementary School, and discovered a man in a sport utility vehicle in the parking lot. He was armed with a .45-caliber handgun loaded with hollow point bullets and had 130 rounds of ammunition in the trunk.

The school was placed on lockdown and the man, Thomas J. Wilkie, 46, of Bear, Del., was arrested. The authorities said his intentions were unclear.