Department of Homeland Security Kristi Noem called the shooting a "tragic situation" before arguing that the shots were "defensive" because that the agent was "fearing for his life."

"This looks like a situation where an individual arrived at the scene to inflict maximum damage on individuals and to kill law enforcement," she said, echoing earlier characterizations by Border Patrol Commander at Large Greg Bovino.

Minnesota Gov. Tim Walz and Minneapolis Mayor Jacbo Frey have disputed those characterizations.

ABC News' Luke Barr asked  what evidence Noem had to make those claims that 37-year-old Alex Pretti wanted to injure or hurt law enforcement and, but she did not answer the question.

-ABC News's Emily Chang

The Gun Owners Caucus released a lengthy statement following the shooting death of 37-year-old Alex Pretti in Minneapolis.

Pretti was a legal gun owner who had no criminal record, the Minneapolis police chief said.

"We are deeply concerned by this morning’s reports that a federal law enforcement operation in Minneapolis resulted in the fatal shooting of 37-year-old Alex Pretti by federal agents," the group said in a statement.

The group added that "despite widespread speculation regarding intent, there has been no evidence produced indicating an intent to harm the officers." The group called for a "full and transparent investigation by both state and federal authorities."

"Every peaceable Minnesotan has the right to keep and bear arms—including while attending protests, acting as observers, or exercising their First Amendment rights. These rights do not disappear when someone is lawfully armed, and they must be respected and protected at all times," the caucus said.

The Minnesota National Guard has been activated for operations securing the Whipple Federal Building and the site of this morning's shooting, "providing depth and capability to state and local law enforcement," Army Maj. Andrea Tsuchiya, a spokesperson for the Minnesota National Guard, said.

Minnesota Guardsmen have been on standby since Jan. 8. Frey said members from those of other agencies will wear reflective vests to be more visible.

"Our community is tired, our officers are tired, our businesses are tired and this additional support is necessary," Minneapolis Mayor Jacob Frey said at a news conference.

-ABC News' Christopher Boccia

The 37-year-old American citizen killed by a Border Patrol agent was identified as Alex Pretti, according to the  AFGE Professional Local 3669, which represents professional employees affiliated with the Minneapolis VA Health Care System.

"[A] member of our union lost their life today, and that alone is devastating. Our hearts are heavy, and we are deeply stricken by this tragedy that has befallen one of our own," the union said.

The American Nurses Association also released a statement saying it is "deeply disturbed" by the Pretti's death.

The ANA is calling for a "a full, unencumbered investigation" and urged findings to be shared with Pretti's family and the public.

The Associated Press reported that Pretti worked as an ICU nurse.