Pittsburgh police officers saw gunfire exploding through shattered glass and ran toward it.

They were heading into the Tree of Life Synagogue in Squirrel Hill, a peaceful place of worship that on Saturday morning became a crime scene in the deadliest attack on Jews in the U.S.

"We're under fire! We're under fire! He's got an automatic weapon and firing at us from the synagogue."

It was one of several radio dispatches from officers that gave a raw look at the chaotic and harrowing mass shooting.

"Every unit in the city needs to get here now!"

As police entered the synagogue, they found victims who were already dead. They got wounded survivors to safety, even as they were still being fired upon, according to emergency dispatches and police reports.

More:What we know now: Authorities identify 11 killed in Pittsburgh synagogue massacre

More:Hate in Pa.: Pittsburgh shooting part of 'white terrorism,' state civil rights watchdog says

The tree-lined streets were soon filled with police vehicles, and tactically trained doctors also responded to treat victims at the scene. Six survivors were wounded, including four officers.

One of the officers had a severed artery, according to Pittsburgh Mayor Bill Peduto. "It could've been much worse, but his partner placed a tourniquet."

Pittsburgh Police, SWAT officers exchange fire with Robert Bowers

In active-shooter drills, city police officers and SWAT officers are trained to put the safety of others before themselves, and they continued to move toward the shooter, determined to keep him inside the building.

Robert Bowers, a 46-year-old Baldwin resident who wrote anti-Semitic posts on a social media site popular among white nationalists, is charged in the shooting deaths of 11 people ranging in age from 54 to 97. He was carrying an AR-15 assault rifle and three Glock .357 handguns, according to police reports.

He screamed "All these Jews need to die" and said "I just want to kill Jews," according to police reports.

But he also took aim at responding SWAT officers, and they returned fire, according to dispatches. Two SWAT officers were hit multiple times, including one who was critically wounded, as they worked to take the alleged shooter into custody.

"They're all heroes," Pittsburgh Police Chief Scott Shubert said. "Who knows what would've happened if he got out in the streets."

Listen to audio from the police scanner as officers responded (story continues below):

Wounded officers get treatment, support

After visiting with one of the injured officers, Peduto said that man was in good spirits. He was planning to meet with the rest of the injured officers Sunday.

Gov. Tom Wolf also spent time with the injured officers.

“I have spent today with first responders, with local leaders, and I am in awe of the bravery of displayed by law enforcement, those folks who help to keep people safe, to aid victims, and to prevent further tragedy," he said.

Officials have not formally released the identities of the four officers injured, but confirmed the wounded were two city policemen and two SWAT officers.

One of the city policemen was Officer Michael Smidga, according to the Pittsburgh Post-Gazette.

A GoFundMe account is set up for SWAT Officer Timothy Watson, who was shot multiple times and critically injured, according to fellow Officer Anthony Seretti.

Cmdr. Jason Lando told the Post-Gazette that Watson was cracking jokes in his hospital room Sunday.

“When bullets are flying and people’s lives are in danger, Tim Matson is the guy you want going through the door,” Lando told the Post-Gazette. “He’s the guy who goes in and calmly gets the job done."

More:Find coverage on The Incline, "elevating the news in Pittsburgh"

UPMC, which owns many hospitals in Pittsburgh and throughout Pennsylvania, said the four officers were treated at its facilities. One officer, whose age is not immediately known, was already released. Three others are at two hospitals:

40-year-old male officer is in critical condition in the intensive care unit at UPMC Presbyterian

55-year-old male police officer is in stable condition at UPMC Presbyterian

27-year-old male officer was released from UPMC Mercy, as of 6 a.m. Monday.

Because of patient privacy laws, hospitals can't give specific information on injuries.

Dispatch reports indicate an officer was shot in the hand, and another was cut on the face by flying shrapnel.

Bowers also was wounded. He is in stable condition and under police protection at a local hospital.

Investigation: Motive 'very clear'

Law enforcement continues to investigate and could be at the crime scene for a week, authorities said. Bowers is set for arraignment in federal court at 1:30 p.m. Monday.

The alleged shooter's motive seems "very clear," as he sought out the synagogue that worked with refugees and immigrants, Peduto said.

Some of Bowers' social media posts on Gab before the shooting accused Jewish people of supporting and enabling mass migration.

"There was a clear message of hate against the Jewish community," Peduto said.

Despite the numerous social media posts, there was nothing that alerted legal authorities to any illegal activity or a mass shooting so "horrific," Peduto said.

"The police response was tremendous. The shooter was armed at a very high level with a lot of ammunition, indicating there could have been a lot more damage," Allegheny County Executive Rich Fitzgerald said. "City and county police stopped what could've been worse. I give our police and first responders unbelievable gratitude and credit."