New York/IBNS/UNI: A gunman riding a scooter without a license plate opened fire in New York's Queens and Brooklyn districts at four people, killing an 87-year-old man and injuring three others, the New York Police Department (NYPD) has said.

"A[n] … individual who took a part himself to shoot four innocent people, tragically killing one … The shooter in this connected incident used an illegal scooter that did not have a license plate to move from one location to the next," First Deputy Commissioner Edward Caban told a briefing on Saturday.

The police have detained a 25-year-old man, who staged a string of shootings in Brooklyn and Queens, and whose motives are still unknown and presumably "random," Joseph Kenny, the assistant chief in the NYPD's Detective Bureau, told a news conference.

"At this time, the video shows that he is not targeting anybody, he is not following anybody. As he is driving on his scooter, he is randomly shooting people," Kenny added.

The investigation into the incident is ongoing.