PITTSBURGH (NYTIMES) - Once again, a gunman entered a house of worship and opened fire.

This time, it was a synagogue in Pittsburgh, where 11 people were killed and six others were wounded on Saturday (Oct 27).

Mass shootings have become a recurring part of American life, and religious institutions a recurring setting. In each case, the shock is compounded by the violence at what is supposed to be a safe space for peace and healing.

Here is a look at what happened in some of the deadliest mass shootings at houses of worship in recent years.

26 killed at a Texas church in 2017

A gunman stormed into a church in Sutherland Springs, Texas, last year and opened fire during a Sunday service, killing 26 people and wounding 20 more.

The authorities said Devin Patrick Kelley, 26, was clad in all black, with a ballistic vest strapped to his chest and armed with a military-style rifle, when he quickly attacked the small church, pausing only to reload.

Kelley died of a self-inflicted gunshot wound.

Law enforcement authorities have said the shooting might have stemmed from a dispute between Kelley and his mother-in-law. She and several others from that side of the family were regulars at the church. But on the morning of the shooting, Kelley's mother-in-law was at home with her grandson.

Among those killed were several children, the pastor's 14-year-old daughter and eight members of a single family.

9 killed at a historic black church in South Carolina in 2015

In 2015, a white gunman killed nine black parishioners at a historic black church in downtown Charleston, South Carolina.

Dylann Roof, a self-radicalised white supremacist, confessed to the killings.

Roof, then 21, entered through an unlocked side door of the Emanuel African Methodist Episcopal Church and took a seat at a weekly Bible study meeting. He had brought with him a semi-automatic pistol that he concealed in a pack on his waist.

When the congregants closed their eyes for a familiar benediction, the sound of gunfire roared through the fellowship hall. Churchgoers dove below tables but Roof kept firing, striking the victims at least 60 times.

Roof was charged with 33 counts, including hate crimes resulting in death. He was found guilty in 2016 and sentenced to death last year.

At Emanuel AME Church, a bedrock of black Charleston that was founded in 1791, doors that were once left open have had to be locked since the shooting. The church has used retired military and on- and off-duty police officers for security.

Mr Willi Glee, a member of the church, said last year that religious spaces offer an added attraction for some attacks because defiling them is the ultimate taboo.

"Everybody expects that a house of worship is a safe place," he said.

6 killed at a Sikh temple in Wisconsin in 2012

In 2012, a Sikh temple in Oak Creek, Wisconsin, was getting ready for Sunday services when an armed man walked in and started firing.

Congregants ran for shelter and barricaded themselves in bathrooms and prayer halls, where they made desperate phone calls and sent anguished texts pleading for help as confusion and fear took hold.

Six people died and three others were wounded.

The gunman, Wade Page, an Army veteran with ties to white supremacist groups, killed himself.

In the years since the attack on the Sikh temple, its worshippers have done their best to move forward. They forgave the assailant "that very day", a priest, Mr Harjinder Singh, has said.

But after the violent attack on the church in Sutherland Springs last year, he said that each new attack on a house of worship brings back the grief.

"Every time this happens, we feel the pain again," he said. "It is like when you have a bandage on your body, and it is ripped away."