LONDON — When British police launched a murder investigation Tuesday into the shooting death of a 17-year-old girl it pushed the homicide rate in Britain's capital to a new, troubling milestone: London's murder rate surpassed New York City's for the first time.

London tallied 15 murders in February and 22 in March, slightly more than New York City for the same two months, which logged 14 in February and 21 in March, according to murder rate statistics provided to USA TODAY by London's Metropolitan Police and the New York Police Department.

Overall London has so far in 2018 seen fewer killings than New York City: 47 versus 54. Both cities have similarly sized populations of about 9 million.

In London, the homicide rate has increased as the British capital experiences a rise in knife-related crime — responsible for 31 deaths this year — while New York City's murder rate has steadily dropped for almost three decades. Of the 54 murders in New York City this year, 32 people died by a firearm.

Last year, New York City police investigated 290 murders, part of the sharp decline that New York Police Department Commissioner James O’Neill has attributed to "precision" policing tactics including building trust with communities in disaffected neighborhoods, targeting the worst offenders and the better use of technology.

In 1990, by comparison, New York City counted more than 2,200 murders, according to police data.

There were 5,738 homicides in the U.S.'s 50 biggest cities in 2017 compared with 5,863 homicides in 2016, a roughly 2.3% reduction, according to a USA TODAY analysis.

Chicago, which has had high levels of violence for years, saw its murder tally dip to 650 in 2017 from 762 in the prior year. But Baltimore is the U.S. city with the highest per capita murder rate in the nation, with nearly 56 murders per 100,000 people in 2017.

In the British capital, there were 134 murders in 2017, a 40% increase over the last three years — excluding deaths in terrorist attacks. London Police Commissioner Cressida Dick said recently she believes social media applications such as YouTube, Snapchat and Instagram are partly to blame for the way they normalize violence.

"There’s definitely something about the impact of social media in terms of people being able to go from slightly angry with each other to 'fight' very quickly," Dick told The (London) Times on Friday. She said online insults and threats "make (violence) faster, it makes it harder for people to cool down. I’m sure it does rev people up."

David Wilson, a criminologist at England's Birmingham City University, said additional factors that could account for London's increased murder rate include surging gang activity among Britain's youth amid poor economic prospects, hollowed-out police resources and staff in the wake of a series of federal budget cuts and a failure by British authorities to adopt a "zero tolerance" approach to relatively low-level crime.

"Britain has never done that," said Wilson, referring to the policy which imposes strict punishments for minor infractions and incivilities, and which some police and criminologists have credited with helping to end New York City's high homicide rate.

Contributing: USA TODAY's Aamer Madhani