University of Connecticut Health Center spokeswoman Carolyn Pennington Thursday confirmed Lanza's DNA was being tested at the request of the state medical examiner's office, CNN reported.
Pennington said the examination isn't intended to identify a specific genetic marker. Results are expected "probably the end of January," she said.
Lanza killed 20 students and six adults at Sandy Hook Elementary School Dec. 14, and then took his own life. Before going to the school, he killed his mother at her home.
CNN Chief Medical Correspondent Dr. Sanjay Gupta said the DNA analysis probably won't yield a genetic explanation for the rampage.
"There's no clear-cut sort of ... genes identified with the types of illnesses, mental illnesses, that may cause this sort of behavior," he said.
There are genetic aspects in many cases of mental illness but they involve large numbers of genes, making it next to impossible to identify genetic cause-and-effect.
Steve Warren, chairman of the Department of Human Genetics at Emory University in Atlanta, told CNN there is "nothing you could look at that would give an answer," to explain behavior.
"We don't know a gene that says 'this person has schizophrenia,'" Warren said. "There's no way to come up with a conclusion that way."