TO Device dumped outside Gracie Mansion During protests in New York City on Saturday, it was confirmed to be an improvised explosive device, according to police.

Two men were detained after at least one of the two devices was ignited during an anti-Islam demonstration led by conservative influencer Jake Lang and a counter-protest outside the mansion, which serves as the residence of the mayor of New York City. At the time it was unclear what the devices were and whether they posed a danger to the public.

An FBI spokesman said the two men arrested in the case are in federal custody, where they are being interviewed by federal agents and the NYPD.

Federal and local authorities are investigating the matter as a possible act of terrorism in part because one of the suspects in custody made direct reference to ISIS in statements to authorities, several people familiar with the matter told NBC News.

New York City Mayor Zohran Mamdani, who is Muslim, was inside the residence at the time of the incident.

“Emir Balat and Ibrahim Kayumi were arrested at the scene yesterday and are in custody in connection with this matter,” NYPD Commissioner Jessica Tisch said in a post on X.

More testing is being done on both devices, the commissioner said.

Officials are working with federal prosecutors on the investigation, Tisch said. It is unclear if charges have been filed against the men.

Mamdani thanked law enforcement officials who acted quickly and said in a statement Sunday that he was working closely with Tisch on the case.

He condemned the original event organized by Lang, whom he described as a white supremacist, as one rooted in bigotry. That kind of hate “has no place in New York City,” the mayor said.

“What followed was even more disturbing. Violence at a protest is never acceptable. The attempt to use an explosive device and injure others is not only criminal, it is reprehensible and the antithesis of who we are,” Mamdani added.

The protest led by Lang in front of Gracie Mansion was called “Stop the Islamic Takeover of New York City, Stop Public Muslim Prayer in New York City” and attracted about 20 people, according to police. But the counterprotest attracted about 125 protesters at its peak.

Both groups were separated in designated areas by police when protests began around 11 a.m. Saturday, but authorities said tensions escalated within an hour.

At 12:15 a protester from the Lang group fired pepper spray at the counter-protesters and was detained, according to Tisch. Then, shortly after 12:30 p.m., Tisch said, an 18-year-old counterprotester “lit up and threw a lit device toward the protest area.”

Both men arrested in the case are from Pennsylvania, where law enforcement officials went to speak with family members and contacts, sources familiar with the investigation told NBC News.

Searches of both their homes and electronic devices are expected, two sources said.

The NYPD’s bomb squad is responding to an area near Gracie Mansion where investigators believe the suspects had parked their car, according to three sources familiar with the matter. They said a search will be conducted to see if there are additional explosives, describing it as a precautionary measure.

The incident occurred halfway the holy month of ramadanthe ninth month of the Islamic calendar, in which Muslims around the world fast from sunrise to sunset. It is intended to be a month of reflection and spiritual connection in commemoration of when the first verses of the Quran were revealed to the Prophet Muhammad.