Thu Hong Nguyen was previously charged with one count of arson, two counts of second-degree assault, two counts of second-degree murder and one count of causing catastrophe.
In October 2015, prosecutors say Nguyen intentionally poured acetone and isopropyl alcohol in the stock room of her KCMO nail salon, lit it on fire and then left the building to burn.
Nhat Pham, a co-owner of the KCMO nail salon, was charged Tuesday with insurance fraud after allegedly lying to an insurance company about his location the night of the 2015 fire.
Responding firefighters Larry Leggio and John Mesh were killed when a portion of the building collapsed. Two other firefighters were seriously injured in the fire.
On Tuesday, Nguyen was charged in Jackson County with another count of arson for allegedly setting another nail salon in Lee's Summit on fire in 2013.
Court documents say Nguyen worked at Nails USA, which her son owned, in the Cedar Creek Mall shopping area at the time. She told police she closed the business early on July 25, 2013, and a short time later, a witness reported a fire in the salon to emergency dispatch.
Investigators said a desk inside the salon -- the station where Nguyen worked at -- caught fire, but it was quickly extinguished, which limited the damage.
In an initial investigation, the Lee's Summit Fire Department classified the fire as accidental due to inappropriate electrical wiring.
But after Nguyen was charged in the 2015 KCMO fire, officials went back to the 2013 investigation and now say it was no accident.
Court documents say two other employees left the building shortly before Nguyen did that evening, leaving Nguyen inside the business alone.
Investigators did several tests to try to reconstruct the 2013 fire, and those tests eliminated all accidental causes of the fire, including a faulty power strip.
As a result, investigators determined the fire had to be intentionally set and believe that Nguyen knowingly damaged the salon and put other people in danger.