Law enforcement has finally caught the mysterious third man in the apparent plot to commit ISIS-inspired terrorism in Boston. Last week, police shot and killed Usaama Rahim in a confrontation in a parking lot, and arrested his nephew David Wright on charges of destroying evidence. Police and FBI had raided a Rhode Island home looking for the suspected accomplice who had been caught on tape allegedly discussing a plot in code, but the man had left. NBC reports this morning that FBI agents arrested Nicholas Rovinski overnight, after questioning him for two days about the plot:

A Rhode Island man was arrested Thursday night in connection with an alleged terrorism that left a Boston man dead last week, the FBI told NBC News.

The man, Nicholas Rovinski, 24, of Warwick, Rhode Island, is the third, unidentified man referred to in court documents with whom Usaamah Abdullah Rahim and David Wright met on a Rhode Island beach Sunday to discuss an alleged plot to kill police officers, multiple law enforcement sources told NBC News. …

The charges against Rovinski are under seal, and the FBI wouldn’t comment on his relationship to Rahim and Wright. The U.S. Attorney’s Office in Boston wouldn’t comment, saying the complaint would be made public before Rovinski appears in court Friday.

NBC’s local affiliate in Rhode Island reported last night that the FBI and police had actually caught up with Rovinski on Tuesday. According to those who live around Rovinski, law enforcement had been in the neighborhood ever since:

NBC News confirmed police and the FBI have been questioning a Rhode Island man since Tuesday. Authorities said he is the person referred to in court documents as having attended a discussion with Rahim and Wright last Sunday “on a beach in Rhode Island, in inclement weather” to talk about Rahim’s plans.

Separately, the NBC 10 I-Team has identified the Warwick man investigated by police as 24-year-old Nicholas Rovinski, who lives a short distance from Gaspee Point with his mother and younger brother on Aspinet Drive. The information was confirmed by a senior law enforcement official with knowledge of the investigation.

“Growing up with him, he was never a threat to anybody, always a good kid, always really nice too,” said neighbor Brandon Waterman.

Other neighbors are now wondering why the police are on the street for the third day in a row.

“If he’s in there, why not arrest him? My conclusion is they don’t have enough evidence,” Donnelly said.

Perhaps that conclusion was a little premature. NBC 10’s report came out prior to the news of the official arrest; the news team’s sources said the investigation as of last night was “not yet complete.” It is now — the FBI plans to arraign Rovinski today, which means they believe they have probable cause to charge him with a crime.

It will be interesting to learn why it took two days of interrogation in Rovinski’s residence to reach that stage, but we probably won’t learn that until the trial itself. Given the nature of the motivation, the FBI must have wanted to know who had been in touch with Rahim and who had radicalized him, and perhaps whether this cell was entirely self-created or connected to other efforts in the US. If that took two days, then either Rovinski was particularly obstinate or he had a lot to say.