Newtown police say the threat was made around 9 a.m., prompting an evacuation.
Lt. Aaron Bahamonde said there was a heightened level of anxiety in town on the anniversary, and the school superintendent decided to cancel remaining classes.
Sandy Hook Elementary School was evacuated this morning while a threat is being investigated by our police dept. Please stand with our community as we attempt to survive another tragic anniversary. #EndGunViolence #SandyHookStrong pic.twitter.com/qNsXADnoPr— Newtown Action (@NewtownAction) December 14, 2018
It's unclear whether the threat was related to the bomb threats made nationwide Thursday.
The school where the shooting happened on December 14, 2012, was knocked down, and a new building was constructed at the same site.
Remembering the victims of the Sandy Hook school shooting:
Moments of silence were observed in Newtown and other places Friday morning in memory of the victims, and bells at a local church were rung at 9:30 a.m.
All town offices in Newtown were closed from 9:30 a.m. to 9:45 a.m. Friday for a moment of silence and period of reflection.
Gov. Dannel P. Malloy has ordered all state and U.S. flags to remain at half-staff in remembrance of the victims. The flags have been lowered in honor of the late President George H.W. Bush.
An interfaith memorial service is scheduled for 7 p.m. at Congregation Adath Israel of Newtown.
Senator Chris Murphy released the following statement on the anniversary:
"Today is an awful day. I'll think a lot about my youngest son, who went off to his first grade classroom this morning, as I remember the precious children and brave educators whose lives were unimaginably taken six years ago. I've had the honor of representing, and becoming friends with, many of the families of the victims. Nothing we do can ever bring those kids back, but we should be inspired by Newtown's efforts to make the world a kinder, more loving place. We can reach out to one another and help those in need. This morning, I'm delivering meals to families in need in New Haven. I hope everyone takes some time to do their own act of kindness - big or small - in their communities.
We will never stop remembering, and honoring, and fighting for the lives senseless lost six years ago. We have to keep going. For me, that means I will never give up trying to change our broken gun laws to keep our kids safe from gun violence. A national movement for stronger gun laws started six years ago today, and today I feel more confident than ever before that Congress will listen and act on changing our gun laws next year. We would not be here without the voices of so many of the family members from Newtown who want us to honor their loved ones with action."
(The Associated Press contributed to this report)
* More Connecticut news
* Send us a news tip
* Download the abc7NY app for breaking news alerts
* Follow us on YouTube