Pennsylvania Gov. Tom Wolf said he was meeting with law enforcement officials at the scene of the rampage shortly after noon Saturday, roughly two hours after the incident occurred. He's believed to be a 46-year-old man.
Local television station KDKA reported that at least 8 people had died, "possibly more".
Pennsylvania's Attorney General Josh Shapiro is saying that the shooter at the synagogue in Pittsburgh 'shooter claimed innocent lives - and injured first-responders - at a baby naming'.
Several Sabbath services and a circumcision ceremony for an infant, attended by dozens of worshipers on three levels of the building, were underway in the predominately Jewish neighborhood of Squirrel Hill at 9:30 a.m. local time when the first shots were fired, congregants said.
Trump spoke to reporters as he left for that event, calling the shooting a "shame" and lamenting that "something has to be done" about "what's going on" in the United States and around the world.
"They didn't and he was able to do things that unfortunately he shouldn't have been able to do", Trump told reporters before getting on Air Force One to fly to Indianapolis for a farmers' convention. Trump also hinted about stiffer laws on the death penalty for such shootings.
Israeli Prime Minister Benjamin Netanyahu said on Saturday he was "heartbroken and appalled" by the shooting.
Squirrel Hill has historically been the centre of Jewish life in the greater Pittsburgh and is home to 26pc of all Pittsburgh-area Jewish households, according to a study from Brandeis University.
A gunman entered a Pittsburgh synagoguge Saturday morning and committed yet another mass shooting, according to police and multiple reports on the scene.
In 2010, Tree of Life Congregation - founded more than 150 years ago - merged with Or L'Simcha to form Tree of Life (asterisk) Or L'Simcha.
"It's a very horrific crime scene - one of the worst that I've seen, and I've been on some plane crashes".
Chattanooga Mayor Berke also tweeted.
The synagogue was preparing for a service at the time police received calls that members of the congregation were barricaded inside.
In a tweet Saturday, Trump encouraged people to shelter in place and says "looks like multiple fatalities".
It said on Twitter: "There is an active shooter in the area of WILKINS and Shady".
Three police officers are among the "multiple casualties" following a shooting overnight.
Jason Lando, a spokesman for the Pittsburgh police department, said officers were still carrying out searches in the Squirrel Hill neighbourhood.
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