BOSTON (CBS) – The Boston Marathon bombings appear to have been driven by the older of the two suspects, according to CBS News.

Tamerlan Tsarnaev, 26, was killed in a shootout with police Friday as investigators closed in on them.

His younger brother Dzhokhar, 19, was captured and remains under heavy guard in fair condition at Beth Israel Deaconess Medical Center. He was shot in the head, neck, legs, and hand.

Tsarnaev is communicating with authorities in writing. According to CBS News senior correspondent John Miller, he can say about one word at a time. Investigators went through public safety questions with him and tried to find out if he is part of a group.

“His account so far is that this was driven by his brother. It was mostly done online in terms of radicalization, finding instructions, and so on and so forth and there’s no international terrorism organization or Mr. Big behind it,” Miller said Tuesday.

“Now, there’s an important caveat there, which is, everything he said or wrote has to be checked out. They’re not taking that at face value. “

Miller said there are still several unanswered questions about the murder of MIT police office Sean Collier.

“The original question is they walked up to that car and appeared they shot the officer in the head unprovoked, that it was an assassination. But why? How did that fit into their plan? The operating theory now in the investigation is they were short one gun. The older brother had a gun. They wanted to get a gun for the younger brother and the fastest and most efficient way they could think of doing it was a surprise attack on a cop, to take his weapon and go. Officer Collier had a locking holster, it’s like a three-way lock. If you don’t know how to remove the gun, you’re not going to get it out. There was apparently an attempt to yank it and they couldn’t get it and left. “

Chilling new details also came out Monday from the victim who was carjacked during Friday’s rampage.

“They basically said we did the Boston Marathon bombing, but the rest of their conversation is in Russian,” Miller said. “Now, the carjack victim doesn’t speak much English and he doesn’t speak any Russian. So, he was debriefed very carefully about ‘what else could you hear they said?’

“He said the only word I recognized was Manhattan. That tripped a lot. That’s why we saw last week stopping the trains from Boston to New York, searching the trains, and the NYPD flipped on it’s network of license plate readers at all of the bridges and tunnels coming into the city and loaded all of the license plates associated with these guys, because if they were headed to New York, they wanted to make sure that that tripped before they got in.”

Miller told WBZ NewsRadio 1030 the brothers told him, “we’re not going to kill you, because you’re not American.”

The brothers then forced the victim to make several stops to pick up bombs and cash before he could escape at a gas station.

Miller said the FBI has taken that victim back along the route to reconstruct the events.

“They’re certainly looking to see if there’s any indicator that they either had an additional stash of weapons or bombs along the way or had help from others.”