New York City detectives investigating this week's mass shooting were interviewing the attacker's associates in his home state of Nevada on Wednesday, as gun safety advocates expressed dismay that he was able to buy a gun there legally last month despite two reported mental health hospitalizations.

New York City has some of the toughest gun laws in the nation. But Monday’s mass shooting showed their limits, security experts said.

The gunman accused of killing four people in a Midtown Manhattan office tower bought the AR-15-style rifle used in the attack from his boss at a Las Vegas casino.

No motive has been announced. A suicide note was found expressing grievances with the NFL and saying Tamura suffered from CTE, a source said. Follow for live updates.

As for security, badges and sign-in lists for visitors became standard fare in New York office buildings after the Sept. 11 attacks nearly a quarter-century ago. Many buildings installed turnstiles between guard desks and building elevators. In recent months, some buildings had stepped up their precautions even more because of the war in Gaza.