When Briscoe and Curtis check out the Lincoln that was impounded, the car was one of many found to have been from Maryland. However the New York State Registration and Inspection stickers are plainly visible on the car's windshield, indicating that this car does not have actually have Maryland plates, but New York plates instead.
It is true that if the same criminal act is carried out in multiple states federal investigators and prosecutors usually take jurisdiction. However in this instance the bombings in Baltimore and New York City, while likely being carried out by the same person, happened five years apart. Because of the amount of time that passed between the two incidents, and because the same person could not be definitively tied to both incidents, they would be treated as separate crimes and would be prosecuted as such, therefore the FBI and US Attorney would not have jurisdiction. This is why Brian Egan, Kevin Geer, was only tried for the bombing in New York, because it is considered a separate criminal act. Even if they could tie him to the Baltimore bombing because the attacks were so far apart he would still be tried separately for each crime.
As revealed in For God and Country (1996) Brian Egan was not the principle perpetrator of the Cacodyl gas attacks, he just planted the bombs, although this episode did establish the "how" of the crime. Egan worked as a truck driver for a chemical company that manufactured the principle component of Cacodyl, namely the arsenic trioxide, it is plausible he stole some of it. But as we find out in the conclusion to the story the man that actually made the bomb and mixed the chemicals used to be in the Army, he was a Army Ranger (special forces) and had EOD (explosive ordinance disposal) training and experience. Creating and safely handling a Cacodyl bomb was well within his skill-set.
Claire, Jill Hennessy, was not interviewing Detective Pembleton, Andre Braugher, at 30:15 he was being interviewed by Jack, Sam Waterston. Claire was interviewing Detective Bayliss, Kyle Secor, Pembleton came into the room Claire and Bayliss were in wearing his coat and hat after he was done being interviewed by Jack.
Cacodyl, the poisonous gas used in the series is extremely toxic that without chemical handling experience and a great deal of luck it would be more likely to kill the person using it than it would anyone else. It is also a highly regulated chemical making it nearly impossible to obtain for anything but the most specific purposes.
The series never reveal the "hows" and "whys" this chemical was chosen and used as it likely would have made an already difficult story even harder to believe,
The series never reveal the "hows" and "whys" this chemical was chosen and used as it likely would have made an already difficult story even harder to believe,
The fact that this crime was plotted and committed as an interstate act, coupled with the number and types of deaths, would have made this a federal case. The FBI and US Attorney's office would have handled the entire matter, with the Baltimore Homicide and NYPD (and their respective DAs) playing a minor role, at best.