New York City homicides climbed by 20 percent and shootings by nearly 25 percent in the first two months of 2015 -- spikes Police Commissioner William Bratton partially blamed on dealers fighting over "marijuana, a seemingly innocent drug that is being legalized around the country."

From New Year's Day to March 1 this year, there have been 54 killings in the city, compared with 45 in the same time period in 2014, according to the latest NYPD data. Shootings have jumped from 121 in 2014 to 151 so far this year, with gunfire causing 75 percent of the homicides, according to the data.

Despite the increase in killings and shootings, overall serious crime has fallen by about 11 percent so far this year to levels not seen since the early 1990s, Bratton said Monday during briefing with reporters.

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"This is not the bad old days roaring back," Bratton said. "This is something we have our arms around."

The commissioner said even with the continued drop in serious crimes, the increase in gun violence and homicides, especially among marijuana dealers, remains a challenge.

"It is ironic that in a city which is a transfer point for huge amounts of drugs . . . heroin, cocaine, hallucinogens, that one drug [that] is actually the causal factor in so much of our shootings and murder is marijuana," Bratton said. "We just see marijuana everywhere when we make these arrests, and get the guns off the street."

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Cops are seeing a big spike in marijuana-related homicides, and many of the people charged with homicide also have a connection to controlled substances, specifically marijuana, said Dermott Shea, the NYPD's top crime analysis official.

"Marijuana is coming up repeatedly," Shea said.

Bratton maintained Monday, as he as done in the past, that the massive decline in stop-and-frisks has no connection to the gun violence and killings. Chief of Detectives Robert Boyce also said that cops weren't hearing from the street that criminals were more emboldened now to carry weapons.

A study done by the NYPD last year, however, and made available to Newsday, showed that in a review of 10 high shooting precincts, about half showed an increase in gun violence as stops declined, while the others saw a drop in shootings.

Criminologists say the link between fewer stops and crime spikes is modest and that a more accurate assessment may be to look at crimes committed in smaller areas than police precincts.

Bratton said that it was important to keep the crime statistics in perspective in the face of historic crime declines.

"The increase in shootings, homicides, when you match up against the previous 20 some-odd years, is still the second- or third-lowest number for the first two months of the year, Bratton said.