The FBI executed raids Sunday on two homes tied to an incident where an improvised explosive device was used during a protest outside the mayoral residence in New York, and federal agents have opened a terrorism probe as arrests were made.

The FBI moved quickly after an explosive device was thrown during a protest in front of Gracie Mansion, the mayoral residence. Agents searched the homes of 19-year-old Ibrahim Kayumi in Newtown and 18-year-old Emir Balat in Langhorne as part of the inquiry. Authorities say the searches followed reports that at least one device was deployed and other devices are under investigation.

The protest began when conservative social media figure Jake Lang held an anti-Islam rally near Gracie Mansion and a larger group of counterprotesters gathered across from the demonstrators. Tensions rose quickly and, according to investigators, at least one counterprotester lit and threw a device toward the opposing crowd. Witnesses described flames and smoke traveling through the air before the device struck a barrier and then extinguished itself a few feet from police officers.

Six people were arrested Saturday in New York City following a dueling protest outside Gracie Mansion, and federal authorities have launched a terrorism investigation.

Two of those arrested are from Bucks County, Pennsylvania, and remain in custody.

On Sunday, FBI agents searched the homes of 19-year-old Ibrahim Kayumi in Newtown and 18-year-old Emir Balat in Langhorne.

The searches came after the NYPD said at least one improvised explosive device was deployed during the protest, with additional devices now under investigation.

“Nothing like this happens around here. So it’s pretty scary,” Newtown neighbor Pamela Pulsinelli said.

Residents reported seeing agents detain a male outside Kayumi’s home.

“Coming out with his arms up and being handcuffed and put into the van. Insane,” Pulsinelli said.

Law enforcement officials say two men confronted Lang during the event and “handled and deployed two explosive devices.” Witness accounts placed flames and smoke in the air as one device flew toward a barrier, where it went out a few feet from uniformed officers. That close call is fueling the federal terrorism investigation and prompted the FBI to take the suspects’ homes under search warrants.

Police described the thrown device as a jar wrapped in black tape, packed with nuts, bolts, and screws — materials that could turn a crude explosive into something capable of causing serious injury or death. The crowd composition on the street split along protest lines, with the anti-Islam demonstrators on one side and the notably larger group of counterprotesters on the other. Officials are treating the hardware and the way it was deployed as central pieces of the ongoing criminal case.

A counterprotester demonstrating against a “Stop the Islamic Takeover of New York City” event Saturday lit and threw a device containing nuts, bolts and screws at the protesting crowd after someone from that group used pepper spray on the counterprotest, police said.

Police are investigating the incident that started late Saturday morning when someone from the anti-Islam protest associated with conservative influencer Jake Lang shot pepper spray into a counterprotesting group near the mayoral residence Gracie Mansion, Commissioner Jessica Tisch said.

Tensions continued to heighten, she said, when someone in the counterprotest lit and threw a device she described as smaller than a football into the protesting crowd of about 20 people.

The device struck a barrier and extinguished itself “a few feet from police officers,” she said. The same person then ran, and another person gave a him a second device, which he then dropped. The devices, which Tisch said were smaller than a football, were wrapped in black tape with nuts, bolts, and screws, as well as a hobby fuse that could be lit. She said it was unclear if they were functioning devices or a hoax.

Officers arrested two people accused of throwing the device along with several others on charges that include disorderly conduct and assault. The federal complaint alleges one of the suspects expressed a desire for the attack to be deadlier than the Boston Marathon bombing, a detail that has heightened the seriousness of the case. Federal and local investigators are coordinating to sort through evidence, interview witnesses, and determine whether additional charges tied to domestic terrorism will follow.

Neighbors described seeing federal agents detain a young man outside one home and take him into custody, a scene that shook the small community. Authorities continue to examine any additional materials found and are looking into whether more devices existed or if those recovered were intended as hoaxes. The investigation remains active, and law enforcement officials say they will provide updates as cases progress and charges are formalized.