A suspect was in custody on Saturday morning after a fatal shooting near the Tree of Life Synagogue in Pittsburgh, where Sabbath services were taking place, according to local officials.

At least 11 people were killed, according to officials. Two suffered injuries that are "critical and serious in nature" and four police officers, three of whom were shot, had non-life-threatening injuries, according to Pittsburgh Public Safety Director Wendell Hissrich.

"It’s a very horrific crime scene. It’s one of the worst that I've seen," Hissrich said. "It's very bad.”

Hissrich declined to say whether children were among the injured but said he believed a service was occurring at the time of the attack.

Officials said the suspect, identified by multiple law enforcement officials as Robert Bowers, 46, of Pittsburgh, was armed with an AR-15-style rifle and multiple handguns.

Social media accounts that appear to belong to Bowers contained posts with anti-Semitic messages and hate speech. It is believed Bowers said, "All Jews must die," before opening fire in the synagogue.

Hissrich said the FBI would be leading the investigation. The Pittsburgh Department of Public Safety tweeted that the shooting would be prosecuted as a hate crime.

Congregants told MSNBC that a circumcision celebration, known as a bris, was taking place in the synagogue this morning.

Pittsburgh Public Safety tweeted on Saturday morning that there was an active shooter in the area around the synagogue and said to avoid the area. Pittsburgh Police Commander Jason Lando said there were multiple casualties.

Jeff Finkelstein, CEO of the Jewish Federation of Greater Pittsburgh since 2004, told NBC News affiliate WPXI that he rushed to the area when he heard about the shooting.

“I just feel horrible for all those people inside,” Finkelstein said. “You know, everyone thinks about Israel in situations like this. It’s just shocking to come back to Squirrel Hill and see something like this.”

Rabbi Chuck Diamond, a former spiritual leader of Tree of Life, told reporters that he never spoke to his congregation about what to do during a shooting, but it's something he thought about.

"There's a lot of anti-Semitism out there and a lot of hate out there," Diamond said. "Sobering that it's touched our community."

Michael Eisenberg, the immediate past president of the Tree of Life Synagogue who lives about a block from the building, told the Associated Press that he was getting ready for services when he received a phone call from a member who works with Pittsburgh's Emergency Services, saying he had been notified through scanner and other communications that there was an active shooter at their synagogue.

"I ran out of the house without changing and I saw the street blocked with police cars. It was a surreal scene. And someone yelled, 'Get out of here.' I realized it was a police officer along the side of the house. ... I am sure I know all of the people, all of the fatalities. I am just waiting to see," Eisenberg said.

He said officials at the synagogue had not gotten any threats that he knew of prior to the shooting. The synagogue maintenance employees had recently checked all of the emergency exits and doors to make sure they were cleared and working.

"I spoke to a maintenance person who was in the building and heard the shots. He was able to escape through one of the side exit doors we had made sure was functioning," Eisenberg told the AP.

Following the attack, the social media platform Gab said in a statement that the site verified an account as matching the alleged shooter's identity.

"Gab took swift and proactive action to contact law enforcement immediately. We first backed up all user data from the account and then proceeded to suspend the account. We then contacted the FBI and made them aware of this account and the user data in our possession," the statement read.

Agents from the Pittsburgh field office of the Bureau of Alcohol, Tobacco, Firearms and Explosives were also responding to the shooting incident.

President Donald Trump said at the National Future Farmers of America Convention and Expo in Indiana on Saturday that "this was an anti-Semitic act."

"You wouldn’t think this would be possible in this day and age but we just don't seem to learn from the past," he said. "Our minds cannot comprehend the cruel hate and the twisted malice that could cause a person to unleash such terrible violence during a baby naming ceremony."

Earlier in the day he tweeted, "Events in Pittsburgh are far more devastating than originally thought. Spoke with Mayor and Governor to inform them that the Federal Government has been, and will be, with them all the way."

Representative Chuck Fleischmann of Tennessee said he is "heartbroken" about what happened in Pittsburgh.

Chattanooga Mayor Berke also tweeted.

Pennsylvania Gov. Tom Wolf tweeted that Pennsylvania State Police were also assisting with the shooting.

"We are still learning details about the shooting at the Tree of Life Synagogue in Pittsburgh but it is a serious situation ... keep the congregants and law enforcement in your prayers," Wolf wrote.

A vigil at the Sixth Presbyterian Church in Squirrel Hill was planned for Saturday night, and another vigil in New York City was scheduled.

Across the country, cities said they would be stepping up police presence at houses of worship, including in New York City, Baltimore and Los Angeles.

In Atlanta, police were also dispatched to synagogues. Peter Berg, senior rabbi at The Temple, told NBC affiliate WPIX that he was “devastated” to learn of the “horrifically tragic shooting” in Pittsburgh.

“I think people are concerned on a lot of different levels,” he said. “One level of concern is: how could this keep happening in our world? How could there continue to be so much gun violence?”

“This is an incident of explicit anti-Semitism,” he said.

Shabbat morning services began at 9:45 a.m. at the Tree of Life Congregation, which is in close proximity to Carnegie Mellon University, according to its website. The Conservative congregation was founded more than 150 years ago and is led by Rabbi Jeffrey Myers, who came to the synagogue in August 2017.

Saturday mornings are when Shabbat services take place in the Jewish faith and are typically the busiest time of the week.

Squirrel Hill is considered a historic Jewish enclave and center for Jewish life in Pittsburgh, and it is considered home to more than a quarter of Jewish households in the Pittsburgh-area, according to a Brandeis University study of the Greater Pittsburgh Jewish community.

More than 80 percent of Squirrel Hill residents said they had some concern or were very concerned about the rise of anti-Semitism. Many also reported incidents of anti-Semitic experiences in the past year, from insults and stereotypes to physical threats or attacks, according to the study.

This is a developing news story. Check back for more information.