Heavily armed police and National Guard soldiers deployed throughout New York as the city of 8.4 million prepared to welcome world leaders at the UN General Assembly on Monday.
The attack happened late Saturday in Chelsea, one of Manhattan's most fashionable districts packed with bars, restaurants and luxury residential buildings.
Police discovered a second bomb planted four blocks away, which was safely defused and is currently being analyzed, officers said.
The bombing came as a jihadist-linked news agency claimed that an Islamic State group "soldier" carried out a stabbing attack in a US mall that left eight people injured late Saturday in the state of Minnesota.
Local police said the attacker "made some references to Allah," but the motive was unclear. The attacker was shot dead by an off-duty police officer. There was no suggestion it was linked to the New York bombing.
"A bomb exploding in New York is obviously an act of terrorism, but it's not linked to international terrorism," New York Governor Andrew Cuomo told reporters on Sunday as he visited the crime scene.
"In other words, we find no ISIS connection, etcetera," said Cuomo in reference to IS, which is based in Iraq and Syria.
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