Was Usaama Rahim just waiting for a bus, minding his own business before getting into a confrontation with Boston police — or was he about to launch a terror plot with other accomplices? Police shot and killed Rahim after he took out a knife and refused to surrender, and police tell WCVB in Boston that investigators needed to arrest him that day to prevent something worse:

Police arrested another man, David Wright, as a suspected accomplice in Rahim’s alleged plot, and say they’re looking for more suspects too. CBS reports that investigators believe the plot was tied to ISIS:

A Massachusetts man is in custody in connection with Tuesday’s shooting of a man police say was radicalized by Islamic State of Iraq and Syria (ISIS) propaganda, reports CBS News correspondent Jeff Pegues. …

Evans says the shooting was caught on nearby surveillance cameras, and the video shows police guns weren’t drawn at first.

“And they kept retreating, verbally giving commands to ‘drop the weapon, drop the weapon,”‘ Evans said. “At some point the individual — proximity came [so] close that the officers were in danger.”

Federal law enforcement sources say Rahim had been on joint terrorism task force radar for several months, but in recent weeks they began tracking him around the clock.

Sources tell CBS News Rahim and Wright were communicating with each other about attacks in the U.S., which is why Wright was arrested. Authorities continue to search for other people both Rahim and Wright were communicating with, the sources say.

In fact, police are confident enough in the video of the shooting to have invited the Islamic Society of Boston to watch the surveillance video of the incident to show that they have Rahim plenty of opportunity to surrender. Their cultural center, which had employed Rahim occasionally in the past as a security guard through a contractor, distanced themselves from him late yesterday. CNN also reports that the attempted arrest came after a “recent change in behavior”:

The ISBCC said that it had had a limited relationship with Rahim. A security firm it uses to provide security at the center hired him as a guard for a month in 2013. Rahim did not regularly pray at the center, nor volunteer, nor serve in any leadership position, the center said.

The FBI-led task force had been watching Rahim and two associates believed to be radicalized by ISIS and other extremist influences, according to a law enforcement official. Rahim had been monitored for at least a couple of years. Investigators were talking to the associates, and various locations in Massachusetts and Rhode Island were being searched, officials said.

The FBI noted a recent change in the suspect’s behavior, including social media threats against police, which prompted agents to try to approach him Tuesday, according to the official.

Wright faces a federal court today for arraignment after his arrest, but the probe isn’t over. Police arrived “in force” in a Warwick, Rhode Island neighborhood, weapons drawn, but didn’t find the suspect — a young man in his mid-20s who has been “acting strangely” in recent months, growing a long beard and worrying neighbors:

Authorities also searched a home in Rhode Island, Tuesday, but would not confirm that was linked to the Roslindale shooting.

Shortly Rahim was killed, federal agents and RI State Police showed up at a home on Aspinet Drive in Warwick. Neighbors say officers went into the house with their weapons drawn, but no shots were fired.

Neighbors say a man in his late 20’s has lived in the house for quite some time. According to them, the man recently grew a beard, and started wearing robes and acting strange. It is unclear if anyone was taken into custody at the home.

The communications of Wright and Rahim appear to have been tracked for some time, so investigators should have at least a few hot leads. However, the shooting probably alerted the rest of the conspirators to go on the lam, which may be what happened in Warwick, assuming they had the correct man in mind in the first place. Police would have much preferred a quiet arrest in order to roll up the rest of this potential terrorist cell. Their job got a lot tougher instead.

Update: Well, at least we know what the knife was for:

#FoxNews #Breaking Fox confirms: terrorists in Boston were plotting to kill officers and behead them.

— Martha MacCallum (@marthamaccallum) June 3, 2015

Unfortunately for Rahim if this is true, the cops brought guns to the knife fight.