The gunman who killed a police officer and three other people at a Midtown Manhattan office building on Monday bought his assault rifle for $1,400 from the man who supervised him at his job at the Horseshoe Casino in Las Vegas, New York police officials said on Wednesday.

The police did not say during their briefing whether the sale of the weapon to the gunman, Shane Devon Tamura, 27, was illegal, but said the seller had been cooperating with investigators and was not being charged in New York.

Mr. Tamura left a suicide note in his Las Vegas studio apartment, where investigators also found anti-psychotic prescription medication, the police told reporters in a briefing.

Mr. Tamura’s note, which a spokesman read to reporters, appeared to be addressed to his parents.

“When I look into your eyes, all I see is disappointment,” he wrote. “I love you mom. I’m sorry.”

The new details emerged as New York Police Department investigators remained in Las Vegas, talking to Mr. Tamura’s co-workers, friends and family. The officers were attempting to learn more about Mr. Tamura’s mental health history and what led him to New York City on Monday afternoon, when he strode into the office building and began shooting.

This is a developing story and will be updated.