Attorney William T. Kennedy said "the families are aware of other people, other persons that witnessed the killing," the Boston Globe reported Friday.
Kennedy represents the families of Daniel Abreu and Safiro Furtado, who were fatally shot in Boston in 2012. Boston police allege Hernandez, 24, was driving the sports utility vehicle from which the shots were fired.
Hernandez has not been charged in the deaths.
Kennedy said at a news conference at his office in Quincy that he had not deposed the alleged witnesses. He said with their testimony and an affidavit from a Connecticut court he would be able to convince a jury to find against Hernandez.
The suit, filed Wednesday, seeks $6 million for each of the families.
A spokesman for the Suffolk County prosecutor declined to discuss Kennedy's claims, saying investigation of the deaths was "active and ongoing."