According to census statistics compiled by Queens College, Kensington is home to more than 3,000 Bangladeshi-born New Yorkers, making it the largest Bangladeshi enclave in Brooklyn. Mian Quadry, a representative of the Bangladesh Muslim Center in Kensington, said he did not know Mr. Ullah, adding, “This is not what and who we are.”
“The Kensington community we have,” Mr. Quadry said, “it’s people who work very hard just to provide for our family.”
The Masjid Nur Al Islam mosque, which Mr. Ullah was said to have attended, sits on the corner of Church and Chester Avenues in Kensington. It was closed on Monday. One man, who has worshiped at the mosque for 15 years and gave his name only as Mohammad, said Mr. Ullah prayed there regularly, especially during Ramadan. Mr. Ullah, Mohammad said, was close to the mosque’s imam and was often seen with him at afternoon prayers. But Mr. Yousuf, the cabdriver, said he had not seen Mr. Ullah at the mosque in the past five or six months.
From 2012 to 2015, Mr. Ullah held a license issued by the city to drive for-hire vehicles, city officials said. The officials were unable to say if Mr. Ullah had driven a yellow cab or for a private service like Lyft or Uber. It was also unclear, they said, if he had his own car or drove for an employer.
Police officials in Bangladesh said on Monday that they had never heard of Mr. Ullah, and the country’s embassy in Washington issued a statement declaring a “zero tolerance” policy toward terrorism. “A terrorist is a terrorist irrespective of his or her ethnicity or religion,” the statement said, “and must be brought to justice.”
Condemnations of the bombing did little to lessen the confusion in Mr. Ullah’s neighborhood.
“Nothing surprises me anymore today,” Mr. Butrico said. “You don’t know who your neighbors are. Can’t trust nobody anymore.”
An earlier version of this story misstated where the Bangladeshi community in Kensington, Brooklyn, ranks among New York City neighborhoods. It is the fifth-largest in the city, and the largest in Brooklyn; it is not the third-largest in the city.