It's not clear exactly why, but New York City's cops are handing out fewer parking tickets. And that means a little less revenue for a city that raked in more than $546 million from tickets last year.

The number of parking tickets issued by New York City police officers in the last week has plummeted by 93% compared to 2014.

Police activity overall has declined in the last two weeks, with arrests down by more than 50%.

Police Commissioner William Bratton said Monday he's investigating whether the change is due to an organized "slowdown" by the department's 34,500 officers, some of whom are furious with Mayor Bill de Blasio after two New York City policemen were shot and killed.

The friction between the mayor and the police department has been growing ever since the city erupted in protests after a grand jury said it would not indict the officer involved in the death of Eric Garner.

"At this juncture, we're just trying to determine what in fact is going on," Bratton said at a news conference Monday.

Total revenue from tickets of all kinds, including minor offenses, totaled $890 million in 2014, according to the Independent Budget Office. That's a sliver of the city's $77 billion budget.

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The decline could be explained by several factors. Officers are "emerging from a mourning period," Bratton said, after two of their own were killed. And thousands of officers have been busy monitoring anti-police demonstrations after the grand jury's decision in the death of Eric Garner.

Bratton also noted that the number of calls made to report crimes has also fallen in the past two weeks, "so we're not in a public safety crisis."

Mayor de Blasio said Monday that he can't speculate how much the drop in tickets would cost the city.

Bratton points out that the trend could actually save money, since police are often paid overtime to appear in court to testify about summonses.