The motive behind the Newtown, Conn., massacre that caused a national debate on gun control and mental health late last year remains a mystery, according to a report by Connecticut’s state attorney. 

The 44-page report released Monday closes the case on Adam Lanza, who killed 26 people, including 20 children, at an elementary school last December, before killing himself. 

“This investigation with the substantial information available, does not establish a conclusive motive,” according to investigators.

The report highlights Lanza’s mental health issues, his easy access to firearms and his obsession with the 1999 Columbine High School Shootings — many details previously revealed.

Sen. Richard Blumenthal (D-Conn.) said the report is a reminder that Congress needs to enact stiffer school security measures, require universal background checks and work to improve mental health. 

“We should not wait for another gun violence tragedy to institute these reforms and other common sense measures. The cost of inaction is too great,” he said in a statement shortly after the report’s release. 

The killings last year immediately launched a push by congressional Democrats and President Obama to tighten gun control laws. 

The Obama administration quickly issued a number of executive actions to address gun violence in January. 

The debate crested in April as the Senate narrowly voted down a compromised proposal to expand background checks and limit gun sales. 

Despite high-profile lobbying by Vice President Biden, an amendment to ban the sale of semi-automatic assault rifles also failed. 

Lanza used a Bushmaster rifle for all the elementary school shootings, according to the report. He killed himself with a “Glock 20, 10 mm pistol.” Lanza used a .22 caliber rifle to kill his mother before making his way to the school. Another handgun was found on Lanza, and a shotgun was found in his car. 

“He had a familiarity with and access to firearms and ammunition and an obsession with mass murders, in particular the April 1999 shootings at Columbine High School in Colorado. Investigators however, have not discovered any evidence that the shooter voiced or gave any indication to others that he intended to commit such a crime himself,” the report concludes. 

The report found that his mother had legally purchased all the weapons and ammunition.  

The report also found that Lanza did not seek help for his “significant” mental issues, and that they affected his ability to live a normal life. But the report stopped short of making any broader conclusions. 

"Whether this contributed in any way is unknown. The shooter did not recognize or help himself deal with those issues," according to the report. 

Polling in September, found that more people blame the failure of the mental health system for mass shootings above easy access to guns. 

Forty-eight percent of people blame the mental health system “a great deal,” while 40 percent blame the easy access to guns, according to a Gallup poll.