The New York City Police Department continues to work with investigators across the country to learn more about the gunman in Monday's deadly Midtown office shooting.

On Thursday, Las Vegas authorities were investigating if shooter Shane Tamura's boss, identified only as "Rick," broke the law.

The NYPD says Rick legally purchased the rifle used in the Park Avenue attack and then sold it to Tamura, who had a concealed carry permit, for $1,400. The question is, was that sale legal?

How to legally sell a gun in Nevada

In order for the sale to be legal under Nevada law, Rick would have had to go to a gun store that is considered a federal firearms licensee. While there, Rick and Tamura would have had to fill out a document called an ATF Form 4473, which records the transaction.

CBS News New York has reached out to the Las Vegas Metropolitan Police Department and the Bureau of Alcohol, Tobacco, Firearms and Explosives to find out if Rick filed that form.

An attorney representing Rick told CBS News New York his client "administered the transfer of the firearm used in the tragedy in New York lawfully complying with Nevada and Federal gun laws." We're still waiting to get confirmation from authorities.

Authorities have said Rick is cooperating with police, and he has not been charged at this time.

CBS News New York spoke to Rick on Wednesday, and he had no comment.

Meanwhile, at last check, police were waiting on a warrant to search the gunman's locker at the Horseshoe Casino, where he worked.  

Federal law exception for gun sales

Las Vegas defense attorney Josh Tomsheck is an expert on Second Amendment issues.

"I think it's very unlikely that, if he did the transfer at a licensed federal firearms licensee, he is probably not exposed to any criminal liability. That's the reality," Tomsheck said.

But he says under federal law, there is an exception.

"If seller is selling to someone he knows to be a habitual drug user, he can't do that. If he thinks he's got a documented mental health history, he can't sell it to him in that circumstance. He's duty-bound to only sell to someone who he believes can legally purchase the firearm," Tomsheck said.

Kris Chanski, CEO of the Las Vegas gun shop The Range 702, says as a licensed dealer, she has the right to deny any sale, no explanation required.

"Let's say I'm talking to you and you want to purchase a firearm. If I'm just feeling like it's not right, I have the right to dealer deny for no reason at all," she said.

Chanski said it's something she's done "many times."

"I think it's our moral responsibility. I think we have a legal responsibility and a moral responsibility, so it's important to me to meet both of those," she said.