Forty-eight percent of respondents said they blame the mental health system a "great deal," a statistic that has not changed since a poll conducted in 2011, after the massacre in Tucson, Ariz., when six people were killed.
Another 32 percent said they blame the mental health system a "fair amount."
The poll was conducted Sept. 17 and 18, after shooter Aaron Alexis went on a rampage in the Washington Navy Yard and killed 12 people.
Forty percent of respondents said they blame easy access to guns "a great deal," down from 46 percent in 2011.
Forty-nine percent said firearm laws should be made stricter, down from 58 percent in December 2012, after the mass shooting at Sandy Hook Elementary School in Newtown, Conn., Gallup said.
Drug use remains the third-highest-ranking factor in gun violence, but is down 5 percentage points from 2011, the poll indicated.
Gallup interviewed 1,023 adults on the phone for the poll. The sampling error was 4 percentage points.