Before July 4th mass shooting, federal officials warned of 'heightened threat environment'

In the weeks before a mass shooting at a Fourth of July parade in a Chicago suburb, federal law enforcement officials had again been on alert because of a "heightened threat environment" fueled in part, they said, by domestic extremists and social upheaval.

Six people were killed and about 24 others were seriously hurt in the mass shooting at the parade in Highland Park, Illinois, according to officials. As of Monday afternoon, police were still searching for the suspected gunman, whom they described as a man between the ages of 18 and 20.

A rifle was recovered, police said, adding that the suspect should still be considered armed and dangerous. It appears he fired from a roof.

A motive has not yet been confirmed. Investigators initially called it a "random act of violence."

The Department of Homeland Security recently indicated that there was real potential for another spate of violence in the near term and in the foreseeable future because of what federal authorities described as extremists and so-called "lone actors" becoming animated by events and issues ranging from Pride Month to the House's ongoing Jan. 6 hearings to controversial Supreme Court rulings.

Homeland Security Secretary Alejandro Mayorkas last month detailed his concerns and his department released an assessment of some of the threats the U.S. was facing.

"As recent acts of violence in communities across the country have so tragically demonstrated, the nation remains in a heightened threat environment, and we expect that environment will become more dynamic in the coming months," Mayorkas said.

"We do expect that the threat environment is likely to become more dynamic as several high-profile events could be exploited to justify acts of violence against a range of possible targets," a DHS official told reporters on a conference call in June.

Targets of domestic violent extremists, the official said then, included public gatherings, faith-based institutions, racial and religious minorities, government facilities and critical infrastructure. The official said they are seeing threats from the "ideological spectrum" of actors but did not specify.

Separately, the head of the FBI's Washington field office and the U.S. attorney for the district put out a recent joint statement that violence would not be tolerated after a man was arrested in the D.C. area for allegedly plotting to assassinate Justice Brett Kavanaugh last month as the high court prepared to overturn Roe v. Wade.

Also last month, police in a small Idaho city arrested 31 people allegedly affiliated with a white nationalist group near a Pride parade. The people associated with the group "Patriot Front" allegedly had shields, shin guards and other riot gear with them, including at least one smoke grenade, authorities said.

In assessments last week, DHS, the FBI and local law enforcement were clear about concerns that they had for major events -- including those in New York City, San Diego and elsewhere -- over the holiday weekend. Those concerns included strikes on cyber infrastructure as well.

"Special events with significant attendance and media coverage … remain an attractive target for foreign terrorist organizations, homegrown violent extremists, domestic violent extremists, lone offenders, and targeted violence," law enforcement said in its assessment, adding: "Terrorists, criminal hacking groups, and other cyber criminals view special events as attractive targets for cyber attacks designed to facilitate short-term financial gain or highly visible, symbolic disruptions."

Law enforcement specifically has been concerned about "lone wolf" assailants that sometimes fly under the radar of authorities and carry out their attacks alone. Outside experts have said that "lone" terminology can elide the racist and neo-nationalist motivations of those attackers.

ABC News' Emily Shapiro and Pierre Thomas contributed to this report.