But a national autism committee cautioned against speculating about a link between autism and violence.
"Autism is not a mental health disorder -- it is a neurodevelopmental disorder," said the Autism Research Institute's Autistic Global Initiative Project. "The eyes of the world are on this wrenching tragedy -- with 1 in 88 now diagnosed, misinformation could easily trigger increased prejudice and misunderstanding."
A former classmate told CNN affiliate WCBS that Lanza "was just a kid" -- not a troublemaker.
"I don't know who would do anything like this," the ex-classmate said, before walking away distraught. "This is unspeakable."
Lanza's father was also at a loss for explanation. He sent his condolences to the families of victims in a statement released Saturday.
"Our family is grieving along with all those who have been affected by this enormous tragedy," Peter Lanza said. "No words can truly express how heartbroken we are. We are in a state of disbelief and trying to find whatever answers we can."
In New Hampshire, relatives of Nancy Lanza released a statement in which they, too, expressed shock and sadness.
"On behalf of Nancy's mother and siblings we reach out to the community of Newtown and express our heartfelt sorrow for the incomprehensible and profound loss of innocence that has affected so many."