Some archive news footage....
... but the principal action...
The Hollywood Reporter
Berg originally thought of shooting Patriots Day at some of the actual locations where the bombings and subsequent manhunt took place, but he found himself facing stiff local resistance. Watertown and the University of Massachusetts at Dartmouth (where Dzhokhar Tsarnaev was a freshman) denied filming requests. Plans to shoot background footage at the 2016 marathon also raised eyebrows. News accounts quoted locals complaining that turning the marathon into a film set was in "poor taste" and the "rankest form of opportunism," while others suggested that Wahlberg donate his salary to the victims.
In the end, the production did shoot some footage at the 2016 marathon, but filmmakers worked hard to keep an open dialogue with the community.
The actual explosion in the movie is a recreation.
The most violent scenes — the bombing and shootout with the brothers in Watertown (Tamerlan Tsarnaev was killed, and Dzhokhar was captured and sentenced to death in April) — were filmed far from Back Bay at a former Navy airfield 20 miles away in South Weymouth.
"It was intense because these were extras who were from Boston, many of whom were there [at the bombing]," recalls Berg. "Every morning we would all get together, all the extras and the crew; they, too, were emotional about what we were filming, the sequences right after the blast. We made it clear that if somebody was uncomfortable, if somebody was emotional or things got to be too much, there were plenty of people on the crew that they could reach out to, and we would deal with that and take care of them."
The Boston Marathon finish line, re-created on an abandoned airfield. - Same source