Less was killed along with police officer Samuel Jimenez and Dr. Tamara O'Neal. Police said O'Neal's former fiance was the gunman. He was killed by responding police officers.
The lawsuit filed Thursday in Cook County Circuit Court accuses Mercy Hospital, owner Trinity Health Inc. and SDI Security Inc. of negligence.
The lawsuit claims the hospital and the security firm failed to issue a Code Silver" alert to warn of an active shooter or to lockdown the building, which would have kept Lopez out the building.
Mercy Hospital officials say they are aware of the lawsuit but can't comment on pending litigation.
In a lawsuit filed Monday against the city and Chicago police, SDI officials deny liability in the shooting.
PREVIOUS COVERAGE
Mercy Hospital victims remembered on 1 year anniversary of shooting
Mercy Hospital shooting victims remembered with vigil
Woman says she was fired from CHA after reporting Mercy Hospital gunman harassed her
Friends, family say goodbye to Mercy Hospital shooting victim Dr. Tamara O'Neal
Visitation held for Mercy Hospital shooting victim Dr. Tamara O'Neal
Mercy Hospital Chicago Shooting: Funeral for fallen CPD Officer Jimenez to be held Monday
Mercy Hospital Chicago shooting: Slain doctor was 'simply the best,' colleague says
Mercy Hospital Chicago shooting: Slain pharmacy resident was 'best and brightest star,' father says
Mercy Hospital Chicago Shooting: Chicago police mourn rookie Officer Samuel Jimenez
Mercy Hospital Shooting: 4 dead, including Chicago Officer Samuel Jimenez and gunman
The Associated Press contributed to this report.