Schools in Newtown, Connecticut were on lockdown after a "called-in threat", Cody McCubbin, secretary of the Newtown Board of Education, told ABC News.
Police earlier said the lockdown was a "precautionary measure."
Police in Newtown, Connecticut, said all schools in the district were in lockdown as a "precautionary" measure.
In December a gunman killed 26 people in a massacre at Sandy Hook Elementary School in Newtown.
Newtown families who had attended the Boston Marathon are safe and well according to CNN:
CNN confirms Newtown families at the race, are ok, not doing interviews right now
Four months after the Newtown school shooting that left 26 dead, US Senators will vote on Thursday on watered down plans for gun control.Read the full story ›
Connecticut law-makers have passed a wide-ranging gun control law in response to Newtown school shooting last December, the Associated Press reports.
The measure ban the sales of large-capacity ammunition magazines and more than 100 weapons that previously had been legal.
Some of the measures which will take effect right immediately include an expansion of the state's assault weapons ban, background checks for all firearms sales and a ban on the sale or purchase of magazines that hold more than 10 rounds.
The bill also addresses mental health and school security measures.
President Obama has said there will be "shame" on himself and the American people if pledges to bring in tougher gun safety laws after the Newtown school massacre go unfulfilled.
More than 3,000 gun deaths have occurred in the US since Adam Lanza shot dead 26 children and staff at Sandy Hook Elementary School in Connecticut in December.
More shocking details of the speed of the attack and the arsenal of weapons Lanza owned have been released today.
But, as ITV News Correspondent Geraint Vincent reports from Washington, DC, Mr Obama still faces great difficulty in translating anger at the massacre into legislation aimed at curbing the public's right to bear the most powerful of arms.
Newtown killer Adam Lanza's choice of assault weapons from his store of guns to carry out the December school massacre underlines why tighter controls are needed, Connecticut Governor Dannel Malloy has said.
"We now know that he left the lower capacity magazines at home," Malloy said in a statement after the previously sealed searches of Lanza's car and home were made public.
"This is exactly why we need to ban high capacity magazines and why we need to tighten our assault weapons ban."
The White House has released a four point plan to strengthen gun safety laws.
Titled "President Obama's plan to reduce gun violence and help protect our kids," it reads:
President Obama: "This is our best chance in more than a decade to take common-sense steps that will save lives." http://t.co/HDnQwio04X
US President Barack Obama has said action must be taken for new gun safety laws in the wake of the Newtown massacre, saying "shame on us if we've forgotten" the pledge made for the school victims.
"We need everybody to remember what we felt less than 100 days ago and make sure what we said wasn't just a bunch of platitudes," Mr Obama said at the White House.
"The entire country was shocked and pledged we'd do something about it and this time it'd be different," he said.
The president said the gun lobby contains "powerful voices" whose "assumption is that people will just forget about" the demand for tighter laws made after the December killings.
But Mr Obama said: "Nothing is more powerful than millions of people calling for change."