Adam Lanza's older brother, Ryan, 24, of Hoboken, N.J., is being questioned by police, said the first official. Earlier, a law enforcement official mistakenly transposed the brothers' first names.
Both officials spoke on the condition of anonymity because they were not authorized to speak on the record about the developing criminal investigation.
The first official said Adam Lanza is dead from a self-inflicted gunshot wound.
According to the second official, the suspect drove to the scene of the shootings in his mother's car. Three guns were found at the scene - a Glock and a Sig Sauer, both pistols - and a .223-caliber rifle. The rifle was recovered from the back of a car at the school. The two pistols were recovered from inside the school.
The official also said Lanza's girlfriend and another friend are missing in New Jersey.
Meanwhile, former Jersey Journal staff writer Brett Wilshe said he has spoken with Ryan Lanza of Hoboken, who told Wilshe the shooter may have had Ryan Lanza's identification.
Ryan Lanza has a Facebook page that posted updates Friday afternoon that read that "it wasn't me" and "I was at work."