"I speak for all of New York when I offer my prayers for the victims and families of this tragedy," Paterson said hours after the gunman shot several people and took dozens hostage. The gunman had already blocked the back door with his car, authorities said.
NBC's Pete Williams cited city and state officials as saying that as many 13 people might have been killed. But Williams cautioned that the information was very preliminary and may change by because police were still searching the building.
Law enforcement officials also believed the gunman was dead, Williams said, but they were not certain how he died.
Two people were seen in handcuffs as they left the building, but Williams said they were not suspects and that police were simply taking extra precautions as people left the building.
Authorities scheduled a news conference for 4:30 p.m. ET.
At least 41 people were in the American Civic Association building at the time of the shooting, The Binghamton Press & Sun Bulletin reported.
The gunman had a high-powered rifle, Mayor Matthew Ryan told the newspaper. Law enforcement sources later told NBC News that several weapons were recovered from the shooting scene.
Police locked down a nearby high school and advised local business owners to stay inside.
Rich Griffith, who works across the street from the hostage scene, said he saw three people carried out of the building on stretchers alive.
At least six hospitalized
Five people with gunshot wounds were being treated at Wilson Medical Center in Johnson City, according to hospital spokeswoman Christina Boyd.
The wounded ranged in age from 20 to their mid-50s, and their conditions ranged from stable to critical, she said.
Linda Miller, a spokeswoman at Our Lady of Lourdes Hospital in Binghamton, confirmed that a student from Binghamton University was being treated in the emergency room.
"The cops are all standing around in the front now. They're still all over the roof for sure," she said. "The SWAT shooters that were to the side of the building look like they're not there anymore."
When the shooting started at 10:30 a.m. ET, people fled to the basement in search of safety. More than a dozen people were hiding in a closet for more than an hour.
Some people were later escorted out of the building, WBGH reporter Sophia Ojeda said.
College student Leslie Shrager told the AP that she and her five housemates were sleeping when police pounded on the front door of their house next door to the shooting scene.
Officers escorted the six Binghamton University students outside, she said, and that's when they learned of the shooting.
"One of our housemates thought they heard banging of some kind. But when you're living in downtown Binghamton, it's always noisy," said Shrager, of Slingerlands, an Albany suburb. "Literally two minutes later the cops came and got us out."The American Civic Association's Web site says it helps immigrants and refugees with counseling, resettlement, citizenship, family reunification, interpreters and translators.
Mary Pat Hyland, who teaches classes at the center, told MSNBC that many of the immigrants served there are from Vietnam and Laos. "We have a very diverse ethnic area," she said.
Binghamton, with a population around 45,000, is about 150 miles northwest of New York City.