WASHINGTON - The Metropolitan Police Department said on Wednesday that it had opened an investigation into whether NBC and David Gregory, the host of "Meet the Press," broke the law when Gregory displayed a high-capacity gun magazine during an interview on Sunday with the executive vice president of the National Rifle Association.

NBC had asked the police for permission to use a high-capacity magazine and "was informed that possession of a high-capacity magazine is not permissible," said police spokesman Araz Alali. "This matter is currently being investigated," he said. "I can't get into any other specifics."

A spokeswoman for NBC declined to comment.

Gregory displayed the magazine, which rapidly feeds ammunition into the chamber of a gun, about 10 minutes into his interview with the NRA's Wayne LaPierre. The host picked it up from the table in front of him and held it in the air as he questioned LaPierre.

"Let's widen the argument out a little bit," Gregory said. "So here is a magazine for ammunition that carries 30 bullets. Now isn't it possible that, if we got rid of these, if we replaced them and said, 'Well, you could only have a magazine that carries five bullets or 10 bullets,' isn't it just possible that we can reduce the carnage in a situation like Newtown?"

LaPierre said he did not believe it would have made a difference. "There are so many different ways to evade that, even if you had that," he said.

In Washington, people who are caught in possession of the type of magazine that Gregory had can face up to a year in prison, said David Benowitz, a criminal defense lawyer.

Prosecutors and defense lawyers often work out a plea agreement in which defendants receive probation and have a misdemeanor charge on their criminal record, Benowitz said. If defendants have a prior criminal record or lose a jury trial, they could face a stiffer sentence.

Benowitz said the accusation from the police that NBC had asked for permission and then went ahead with showing the magazine "didn't help Gregory's case."

NBC was the only network to have a televised interview on Sunday with LaPierre, who held a nationally televised news conference on Friday to address the issue of gun control in the wake of the school shooting in Newtown, Conn.