District of Columbia police say they warned NBC that having a high-capacity gun magazine in the city was against the law before reporter David Gregory brandished what he said was a 30-round magazine on "Meet the Press."

Police are investigating whether Gregory, 42, violated D.C. gun laws during the program, which was taped in Washington and aired Sunday.

D.C. police spokeswoman Gwen Crump said police discussed the use of the clip with NBC before the program was taped.

"NBC contacted MPD inquiring if they could utilize a high-capacity magazine for their segment. NBC was informed that possession of a high-capacity magazine is not permissible, and their request was denied," according to a statement released by Crump. "The matter is currently being investigated."

In addition to contacting D.C. police, NBC also called the Bureau of Alcohol, Tobacco, Firearms and Explosives on Friday to find out if Gregory could legally display the magazine, according to an ATF source.

The federal law enforcement officer explained to the news program that the agency wasn't responsible for enforcing D.C.'s laws but offered to make some checks. The ATF official then mistakenly reported back to NBC that an empty magazine was not illegal, the source said.

D.C. police spokesman Officer Araz Alali said Wednesday that he could not get into any specifics about the investigation.

NBC officials did not immediately respond to a request for comment.

During the "Meet the Press" interview, Gregory asked National Rifle Association CEO Wayne LaPierre about the deadly shooting at Sandy Hook Elementary School in Newtown, Conn., that left 27 dead, including 20 children.

"Here is a magazine for ammunition that carries 30 bullets," Gregory said while holding up a clip. "Now, isn't it possible if we got rid of these ... that we could reduce the carnage in a situation like Newtown?" It's unclear whether the magazine that Gregory displayed was real or just a prop.

LaPierre said reducing ammunition capacity wouldn't make any difference.

"A gun is a tool. The problem is the criminal," LaPierre said.

D.C. law forbids possession of "any large capacity ammunition feeding device regardless of whether the device is attached to a firearm." A person found guilty can face a fine of up to $1,000 or a year behind bars.

Gregory is on vacation and will not host this Sunday's edition of "Meet The Press," according to NBC officials.