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Eleven people were dead at a conservative Pittsburgh synagogue Saturday after a 46-year-old man, armed with an assault rifle and three handguns, burst into morning services firing shots and calling out anti-Semitic statements, law enforcement officials said.

FBI officials said the suspect, Robert Bowers, was not known to law enforcement before Saturday and was believed to have acted alone, but that agencies remained on high alert as Pittsburghers planned nighttime vigils to honor the victims. Bob Jones, special agent in charge of the agency's Pittsburgh office, said were it not for the fast action by city and county law enforcement, the death toll would have been "much worse."

"The actions this person took today were hateful," Jones said at a Saturday afternoon news briefing. "This is the most horrific crime scene I've seen in 22 years at the FBI."

President Trump, in Indianapolis for a farming convention, said Saturday afternoon that the nation would unite against the tragedy. Earlier in the day, he suggested that if the synagogue had armed guards, the killings might have been prevented.

"With one unified voice we condemn the historic evil of anti-Semitism and every other form of evil," the president said. "And unfortunately evil comes in many forms and we come together as one American people."

Federal authorities said they were treating the shooting, reported to emergency dispatchers at 9:54 a.m., as a hate crime.

"This was an unspeakable act of hate," said Scott W. Brady, the U.S. Attorney for Western Pennsylvania. "Please note that justice in this case will be swift, and it will be severe."

Six people were injured in the incident at the Tree of Life synagogue, including four police officers, of whom three were shot, said Wendell Hissrich, director of Pittsburgh's Department of Public Safety. Among the victims was a 70-year-old man who was shot multiple times in the torso and was in critical condition, officials said.

Law enforcement officials were joined at the news briefing by Gov. Wolf, who pledged unity and called for action.

"Any attack on one community of faith in Pennsylvania is an attack on every community of faith in Pennsylvania," the governor said. "We must take action to prevent these tragedies in the future. We simply cannot accept this violence as a normal part of American life."

In Philadelphia, organizers were planning a vigil of solidarity in Rittenhouse Square, scheduled for 8 p.m., while another was scheduled for 7 p.m. at Congregation Tifereth Israel in Bensalem.

The suspect in the shooting was brought into custody after he had been inside the synagogue for 20 minutes, the FBI's Jones said. But the scene remained chaotic afterward in the western Pennsylvania city.

"There are multiple casualties. Neighbors are advised to shelter in place and stay in their homes and lock their doors," Pittsburgh Police Commander Jason Lando told the Pittsburgh Post-Gazette shortly before 11 a.m.

What happened

Shortly before 11 a.m., the Pittsburgh Public Safety Twitter account warned of an active shooter in Pittsburgh's Squirrel Hill neighborhood. The gunfire erupted at Tree of Life during a baby-naming ceremony, according to the Post-Gazette.

The alleged shooter made anti-Semitic statements during the shooting, CNN reported.

In an update around 1:30 p.m. sharing a hotline number for victim family members, Pittsburgh Mayor Bill Peduto said there were "multiple fatalities," with six wounded, including four police officers.

Gov. Wolf announced that a shooter was in custody shortly before noon Saturday.

"It sounded like a loud crash in the hallway," Stephen Weiss of Squirrel Hill told the Post-Gazette, who was at the synagogue Saturday morning.

Tyler Pepper, 21, reported hearing several gunshots around 9:45 a.m., the paper reported.

"It was kind of a surreal moment," he said, later adding that the gunshots sounded like a long gun.

What’s known about the suspect

The gunman, who was barricaded inside the synagogue, then surrendered to police and was taken to the hospital, where he was listed in fair condition with multiple gunshot wounds, Hissrich said.

The Post-Gazette reports that local officers and agents from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives arrived at a small apartment building on the same street where Bowers may have previously lived according to online records. Allegheny County's bomb squad also arrived at the scene.

What’s known about the synagogue

Tree of Life is more than 150 years old, according to its website, and calls itself a "conservative congregation" that is "also progressive and relevant to the way we live today."

"From our warm, inviting and intellectually stimulating atmosphere to our fun adult, children and family programs, it's the perfect environment to grow a strong faith rooted in tradition," it said. A Shabbat service is held at 9:45 a.m. on Saturdays.

The rabbi, Hazzan Jeffrey Myers, is a Rutgers University graduate who previously worked at a synagogue in Ventnor, N.J., now called Shirat Hayam.

Local response

Gov. Wolf arrived in Pittsburgh early Saturday afternoon. An earlier update from the governor noted that state police were assisting local officials.

"These senseless acts of violence are not who we are as Americans," Wolf said in a statement. "My thoughts right now are focused on the victims, their families and making sure law enforcement has every resource they need."

Other state officials, including Attorney General Josh Shapiro and Sen. Pat Toomey, took to Twitter to say they were monitoring the events in Pittsburgh as they unfolded.

"Synagogues & places of worship are safe havens where communities come together to celebrate, pray, & reflect," Toomey wrote. "The horrific events that transpired at Tree of Life Synagogue are heartbreaking."

Philadelphia police said they are monitoring synagogues and other places of worship in the area. The department will deploy officers to places of worship across the city as a precaution. It is not aware of any current threats to Philadelphia and encourages citizens to contact 911 with any suspicious information.

Political and national response

President Trump said he would travel to Pittsburgh in the wake of Saturday morning's fatal mass shooting, but did not specify when he will make a visit.

The president called the events "far more devastating than originally thought." He was in Indianapolis as the keynote speaker for the Future Farmers of America convention, according to Fox News.

Speaking to reporters, Trump speculated that an armed guard might have been able to stop the shooting.

Staff writer Amy S. Rosenberg contributed to this article.