Hummels and two others were shot following a mediation session involving a lawsuit between Fusion Contact Centers, a telephone call center service, and Arthur Harmon, 70 -- the alleged gunman who subsequently took his own life in Mesa, Ariz.
The head of the call center, Steve Singer, 48, was killed and a bystander, Nicole Hampton, 32, was wounded.
Athia Hard, a spokeswoman for Osborn Maledon, the law firm where Hummels worked, said Hummels was declared brain dead and taken off life support Thursday night, KPHO-TV, Phoenix, reported.
"We are devastated at the loss of our beloved friend," the firm said in a statement Thursday. "Our deepest sympathy and support pour out to his wife, Dana, and their two children. The trust and affection Mark inspired in every reach of our law firm and with his clients are a lasting testament we will always cherish."
Harmon was found dead of a self-inflicted gunshot wound Thursday in a Mesa parking lot, police said.
A man who once worked with Harmon told KTVK-TV, Phoenix, Harmon had "exhibited behavior in the past that would seem to be a little bit hostile and erratic. I think that those should have been a little bit more of a warning sign."