Imam Maulana Akonjee, 55, and his assistant, 64-year-old Thara Uddin, were shot at point-blank range just before 2 pm (1800 GMT) Saturday in the Ozone Park neighbourhood in the borough of Queens, police said.
The motive is unknown and no arrests have been made, the authorities said.
“There is nothing in the preliminary investigation to indicate that they were targeted because of their faith,” police told journalists.
However, Muslim community representatives pointed to growing Islamophobia and anti-Muslim sentiment fueled by a series of deadly attacks in the United States and abroad as well as hostile statements by Republican presidential candidate Donald Trump, including his proposal for a temporary ban on Muslims entering the country.
Security video footage of the brazen attack shows the gunman approach the two men from behind at the corner of Liberty Avenue and 79th Street, a few blocks from the Al-Furqan Jame Masjid mosque.
The victims, dressed in traditional Muslim garb, reportedly left the mosque following Saturday afternoon prayers.
After the attack witnesses saw the gunman flee the scene with a gun, police said, adding that the surveillance video appeared to show a man wearing shorts and a dark polo shirt.
A sketch of the suspect released early Sunday showed a dark-haired, bearded man wearing glasses. Police said witnesses described him as having a medium complexion.
Akonjee was carrying more than $1,000, police said, but noted that the attacker did not take the money.
Both victims were taken to nearby Jamaica Hospital with gunshot wounds and were pronounced dead there.
The suspect was still at large Sunday and the investigation is ongoing, police said.
The culturally diverse, working-class area where the victims were killed, on the border between Queens and Brooklyn, is home to many Muslim families from Bangladesh.
The mayor’s office said the New York Police Department was exploring all potential motives, including the possibility of a hate crime.
“While we do not yet know the motivation for the murders, we do know that our Muslim communities are in the perpetual crosshairs of bigotry,” Mayor Bill de Blasio said in a statement.
“Our city was stung by violence that devastated a congregation and unsettled a community. When religious leaders are targeted, we all bear the pain those in Ozone Park feel most personally today.”
Muslim community representatives condemned what they described as a toxic climate of hatred.
“Please, read my lips. This is a hate crime, no matter which way you look at it,” said Kobir Chowdhury, who heads the nearby Masjid Al-Aman mosque in Brooklyn.
“It’s hate against humanity, it’s hate against Muslims, these are Islamophobes who are causing these kind of troubles.”
Hundreds of local residents rallied near the crime scene under the tracks of an elevated metro line late Saturday, chanting, “We want justice!”
During a vigil outside the Al-Furqan Jame Masjid mosque, the faithful prayed, heads bent and palms facing the sky.
“Imagine your father gunned down for no reason, and then let that feeling, let that motivate you to come out of your silence,” Afaf Nasher, director of the New York chapter of the Council on American-Islamic Relations (CAIR), said at a tense news conference in front of the mosque.
“When we stay silent we allow crimes to continue to occur,” she said.
“So every single one of us shares in this responsibility. And let’s not forget the victims who are essential to all of this.”
Imam Akonjee had moved to the United States from Bangladesh two years ago, US media reported.
“He would not hurt a fly,” his nephew Rahi Majid, told the New York Daily News. “You would watch him come down the street and watch the peace he brings.”
US Representative Nydia Velazquez tweeted that she was “horrified” by the shooting.
“All NYers must stand united in condemning acts like these,” she said.
The neighbourhood’s city council representative Eric Ulrich added: “When a religious leader is killed in broad daylight on the streets of Queens, we must come together as a community and demand justice!”
CAIR announced plans to give a $10,000 reward to anyone with information on the attacker that could lead to his arrest and help determine a motive.
“We hope the offer of a reward will lead to the arrest and conviction of the individual who perpetrated this heinous crime,” said the group’s executive director Nihad Awad.
CAIR plans to hold a funeral prayer service for the slain men on Monday.
Last year, hate crimes against Muslims and mosques tripled across the country following extremist attacks in Paris and San Bernardino, California, according to The New York Times.