Department of Homeland Security Secretary Kirstjen Nielsen called the mass shooting in a Pittsburgh synagogue on Saturday “a pure act of evil” and noted that DHS officials had previously visited the house of worship to give advice to its staff on a possible active shooter scenario.

“This was a pure act of evil,” Nielsen said during an appearance on “Fox News Sunday.” “You’ve heard that from the president and vice president yesterday, that’s what it is. We all condemn this in the strongest terms possible.”

Nielsen added that DHS officials in March had conducted a site visit to the Tree of Life Synagogue with the department’s protective security adviser in the area, but noted the difficulty in advising people on how to handle an active shooter situation.

“Each individual location, each individual event is slightly different and that’s why the planning and the training is so important,” she said. “In such events there is rarely time to think through roles and responsibilities; the response has got to be automatic and that’s where the drills and the workshops that we conduct come in.”

On Saturday morning, Robert Gregory Bowers killed eight men and three women inside the Tree of Life Synagogue in Pittsburgh’s Squirrel Hill neighborhood during worship services before a tactical police team tracked him down and shot him, police said in the affidavit, which contained some previously unreported details on the shooting and the police response.

Calls began coming in to 911 from the synagogue just before 10 a.m. Saturday, reporting "they were being attacked," the document said. Bowers shot one of the first two officers to respond in the hand, and the other was wounded by "shrapnel and broken glass."

A tactical team found Bowers on the third floor, where he shot two officers multiple times, the affidavit said. One officer was described as critically wounded; the document did not describe the other officer's condition.

Two other people in the synagogue, a man and a woman, were wounded by Bowers and were in stable condition, the document said.

Bowers told an officer while he was being treated for his injuries "that he wanted all Jews to die and also that they (Jews) were committing genocide to his people," the affidavit said.

Bowers was charged late Saturday with 11 state counts of criminal homicide, six counts of aggravated assault and 13 counts of ethnic intimidation in what the leader of the Anti-Defamation League called the deadliest attack on Jews in U.S. history.

Bowers was also charged Saturday in a 29-count federal criminal complaint that included charges of obstructing the free exercise of religious beliefs — a federal hate crime — and using a firearm to commit murder. U.S. Attorney General Jeff Sessions said the charges "could lead to the death penalty."

Along with the condemnation from Nielsen and the president, the shooting drew expressions of sympathy from politicians and religious leaders of all stripes across the globe.

Pope Francis led prayers for Pittsburgh on Sunday in Rome at St. Peter's Square.

"In reality, all of us are wounded by this inhuman act of violence," he said. He prayed for God "to help us to extinguish the flames of hatred that develop in our societies, reinforcing the sense of humanity, respect for life and civil and moral values."

German Chancellor Angela Merkel's spokesman quoted Merkel on Twitter as offering her condolences and saying that "all of us must confront anti-Semitism with determination — everywhere."

Fox News' Chris Wallace and The Associated Press contributed to this report.