Barack Obama is offering the Connecticut town grappling with the aftermath of a devastating school shooting "the love and prayers of a nation".
The US president says he is mindful that words cannot match the depth of the sorrow in Newtown, Connecticut, after 20 children and six adults were killed at an elementary school on Friday.
Mr Obama is telling the families of the victims they are not alone in their grief, adding that people all across the country have wept with them.
The president is speaking at a vigil following private meetings with the families and first responders.
This is the fourth trip of Mr Obama's presidency to a grieving city in the aftermath of a mass shooting. In his remarks, he will be addressing not only the residents of Newtown, but also a stunned nation.
Authorities say gunman Adam Lanza shot his mother in the head four times then drove to Sandy Hook Elementary School with enough ammunition to kill every student. He killed 26 people then shot himself as police arrived.
Lanza blasted his way into the school building in Newtown and used a high-powered rifle to kill his victims, including the principal and school psychologist who tried to stop him.
Connecticut governor Dannel Malloy said Lanza shot himself after about 10 minutes of shooting as first responders entered the building.
Investigators said Lanza was carrying an arsenal of hundreds of rounds of especially deadly ammunition - enough to kill just about every pupil in the school if given enough time, raising the chilling possibility that the bloodbath could have been far worse.
The chief medical examiner said the ammunition was the type designed to break up inside a victim's body and inflict the maximum amount of damage, tearing apart bone and tissue.
His 52-year-old mother Nancy was found dead in her bed, shot four times in the head with a .22-calibre rifle. The killer then went to the school with guns he took from his mother, got inside by breaking a window and began blasting his way through the building.
All the victims at the school were shot with the rifle, at least some of them up close, and all were apparently shot more than once, chief medical examiner H Wayne Carver said.
There were as many as 11 shots on the bodies he examined. Lanza died of a gunshot wound to the head from a 10mm gun.
Lanza visited an shooting range before the attack in Connecticut but investigators have not concluded whether he practised shooting there.
Ginger Colburn, a spokeswoman for the Bureau of Alcohol, Tobacco and Firearms, would not identify the range in question or say how recently he was there.
She said investigators had concluded that Lanza's mother Nancy visited shooting ranges several times.
Ms Colburn said it is still not clear whether she took her son to the range or whether he ever fired a weapon there.