BROOKLYN - The mother of the Washington Navy Yard gunman has broken her silence for the first time since her son killed 12 people on Monday.

Cathleen Alexis spoke from her home in Brooklyn, saying that she doesn't know why her son, 34-year-old Aaron Alexis, did what he did and that she is "so, so very sorry."

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Law enforcement sources revealed that the former Navy reservist was being treating for mental illnesses prior to the fatal rampage that also wounded eight people. They say he was receiving treatment from the Veterans Administration for paranoia, a sleep disorder and for hearing harassing voices.

Information on Alexis' mental problems has reignited a volatile national debate on gun control. President Barack Obama reiterated on Tuesday the need for tougher background checks for guns, but advocates in the Senate say they're several votes shy of passing any legislation that would better prevent felons or the mentally ill from buying weapons.