The New York Democrat also called on Congress to pass new legislation to improve the background check system he calls potentially "flawed."
Information from federal and Georgia state officials show the gunman, John Russell Houser, would have passed a background check for his 2014 gun purchase even if he had been involuntarily committed for mental health issues more than six years ago.
That's because Georgia, where Houser was the subject of a mental health evaluation in 2008, removes mental health records from the federal database used to conduct background checks after five years.
In 2008, Houser was the subject of a legal dispute with his family, which sought a protective order against him because of threats he made. A judge issued an "order to apprehend," which required a mental health evaluation of Houser.
However, court records reviewed by CNN do not show what the result of that evaluation was. And there was no order for involuntary commitment issued by a judge, according to Muscogee County Probate Judge Marc E. D'Antonio, the Georgia county's chief clerk at the time.
"Had there been adjudication for the need for involuntary treatment, it would have been reported to the (Georgia Bureau of Investigation)," he said.
An involuntary commitment order would have prohibited Houser from purchasing firearms anywhere in the U.S. during the time that information was in the federal background check system.
But because Georgia would have expunged the information from the national database, Houser could have legally bought a firearm even if he had been institutionalized in 2008. Federal law enforcement officials said the state is the only one that expunges the information after five years.
Houser legally bought the gun last year that he used in the theater attack from a Phenix City, Alabama, pawn shop, according to law enforcement officials.
Probate Court Judge Betty Cason, who issued the "order to apprehend" for Houser, told CNN she feels for the victims of the theater attack, saying "in those parents' hearts and minds, they think I could have prevented them from being killed."
She also defended the order to apprehend, saying she "did what she was supposed to do and the system didn't fail" because of concerns over civil rights.
Earlier this week, Schumer was flanked by family members of those killed by gun violence as he called for improved background checks. Schumer said he hopes that a formal FBI investigation will "provide Congress with more factual data to incent action on ensuring background checks prevent dangerous people from obtaining guns."
In a letter Wednesday to FBI Director James Comey, Schumer noted errors in the FBI's handling of the background check of Dylann Roof that allowed him to legally purchase the handgun he eventually used to murder nine people in Charleston, South Carolina, last month.
Schumer wants to know "whether problems in Georgia or any other state's record keeping and submission inhibited the ability for the NICS system to function fully; and whether there are inconsistencies in the submission of records for people with mental health issues across states."