He said later: "This is the nightmare scenario."
Not long after the blast, police said they had found another possible explosive device just blocks away. This device appeared to be similar to a pressure cooker and had wiring on it, according to a law enforcement official who spoke on the condition of anonymity to discuss the ongoing investigation. Pressure cookers were used in the two bombs detonated at the Boston Marathon in 2013.
The bomb squad safety removed the second device, New York police tweeted.
Police officers, firefighters and other first responders had rushed to the scene of the blast, which closed a major roadway and forced people out of nearby buildings.
New York Mayor Bill de Blasio said that the explosion was "intentional," but noted that few details were available because the investigation was in its early stages.
"We know there was a bombing," de Blasio said. "We know it was a very serious incident, but we have a lot more work to do to be able to say what kind of motivation was behind this."
He also echoed his earlier comments from that there was no specific, credible threat against New York from any terrorist group.
The FBI and NYPD are trying to understand what motivation lay behind the blast.
"We will look at individuals. We will look at associations. We will look at social media," William Sweeney, Assistant Director in Charge of the FBI New York Field Office said.
"Until we have that information it is not fair for us to give a conclusion [as to motivation]," DeBlasio said. "Let the law enforcement experts draw the conclusions."
Police Commissioner James O'Neill, whose first day in the position was Saturday, told reporters on Sunday that police were still gathering evidence.
"We did find some components indicative of an IED. . . that went off intentionally," O'Neill said.
The New York Police Department's counterterrorism bureau responded to the explosion, along with officials with the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives.
While O'Neill said authorities were still trying to determine what, precisely, exploded, the NYPD counterterrorism bureau posted a photo online showing what appeared to be a dumpster or garbage container mangled by a blast:
Police in New York also reported shortly after 11pm that they had found the "possible secondary device" a few blocks away from the Chelsea explosion scene, on 27th Street between Sixth and Seventh avenues. Police directed people away from that intersection, and one officer could be heard telling pedestrians that "there is a possible explosive" in the area.
Not long after midnight, police said in a statement that they were asking people in the area of this potential explosive to move away from their windows "until we clear the suspicious package," although officers did not fully evacuate the area.
According to the Associated Press, the second device was removed with a robot and taken to the department firing range in the Bronx.
Reports of another suspicious package at 28th Street and Fifth Avenue turned out to be a false alarm - the package was only garbage, J. Peter Donald, a police spokesman, posted on Twitter around 12.30am on Sunday.
The Chelsea explosion occurred about 11 hours after a pipe bomb exploded in a Jersey Shore garbage can, shortly before a scheduled charity 5K race there benefiting Marines and Navy sailors.
Authorities have said they have not yet found anything connecting the Chelsea and New Jersey incidents but are continuing to investigate.
On Sunday, Cuomo said the "bombs" in Chelsea "appear to be different" from those discovered earlier in the day in New Jersey, though the devices from both states were being shipped to the FBI lab in Quantico for further assessment. He said the devices in New Jersey were "pipe bombs," and those in Chelsea were not. Cuomo said agents in New York were in touch with those in New Jersey.
Cuomo said officials "have no reason to believe at this time that there is any further immediate threat," though the National Guard and the New York State Police were deploying an additional 1000 personnel to bus terminals, airports and subways "just to err on the side of caution".
Cuomo said the blast in Chelsea "caused significant property damage," though he a nearby subway station was structurally safe. He said law enforcement would work diligently to identify and locate those responsible.
"Whoever placed these bombs, we will find, and they will be brought to justice," Cuomo said.
A spokeswoman for Lenox Health Greenwich Village, which treated nine of the victims, said patients were treated for minor injuries that included lacerations to the head and body. Other hospitals declined to provide specifics about the extent of patients' injuries from the explosion.
Soleil Filomena, 64, was leaving a convenience store at Seventh Avenue and 23rd Street when she heard the explosion.
"It was so loud it just went through my whole body," she said. "People started running up 23rd Street, and I started running with them."
Filomena said she saw a "big black cloud in the sky." After the explosion, she said her "ear was ringing for 15 minutes".
When Keith Salomon of Delaware felt the explosion, he was having dinner a block and a half from the site of the blast. His chair and table shook, he said, and he saw people being taken away in ambulances.
"We didn't know what it was and so, at first, we just kept eating," said Salomon, 52, who was visiting his son in the city. "But then we realised something was wrong."
