The suspect was identified as David Williamson, 44, who died on Monday for his gunshot wound. He is reportedly the husband of Mejilla's receptionist Shawna.
The report, citing witnesses, said Williamson showed no abnormal behavior when he walked into the clinic on Thursday noon.
Jessica Koch, an office manager in the same plaza as the victim's clinic, saw Shawna Williamson run out of the clinic after the shooting and shout "my husband" and "shot" before calling for 911, according to The Star.
A friend of the victim told The Spectator that Williamson may have been jealous of his wife's relationship with Mejilla.
Mejilla, described by patients as generous, had given Mrs. Williamson gifts her husband was unable to afford due to his inability to find work due to his slipped disc, according to a separate report on the The Hamilton Spectator.
The Filipino doctor had previously treated Williamson for his back injury.
Friends, former clients, and colleagues of Mejilla lamented his death as he had regarded his patients as friends, cut them deals when they could not afford treatment, and, according to a fellow chiropractor, "helped countless others without asking for anything in return."
Supporters left flowers and candles in front of the clinic in the subsequent weekend after it was no longer blocked off by police.
Mejilla left behind five children from a previous marriage. —Rie Takumi/KBK, GMA News