By Jessie Yeung, CNN

(CNN) — Investigators are combing through evidence in the wake of New York City’s deadliest mass shooting since 2000, trying to figure how a gunman came to unleash an assault-style rifle in Midtown, fatally shooting four people – including a police officer – and injuring one before killing himself.

The shooting at 345 Park Avenue, a glossy skyscraper near 52nd Street and blocks from busy tourist sites like Rockefeller Center and the Museum of Modern Art, erupted Monday as workers filed out of offices during the busy evening commute. The building houses global firms, including investor Blackstone and the National Football League, with which the gunman – a competitive football player in his youth – had grievances, a law enforcement source said.

It is among at least 25

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