The high-flying protesters who flouted U.S. Bank Stadium security at the last Minnesota Vikings game of the season got out of jail without charges Monday, while those responsible for protecting the $1.1 billion structure had little to say about how the trio pulled off the stunt.

The protesters, who were booked into the Hennepin County jail on Sunday, were out within 24 hours, but they could still face charges after the investigation is complete, deputy Minneapolis City Attorney Mary Ellen Heng said.

The three are Karl Mayo, 32 and the son of former Green Party Minneapolis City Council Member Dean Zimmermann; Sen Holiday, 26, and Carolyn Feldman, 27. Mayo and Holiday scrambled over a wall on a public concourse to reach the ladder on one of the two main steel trusses supporting the stadium roof. From there they climbed the ladder to the catwalk and dropped down to unfurl a giant banner protesting the Dakota Access oil pipeline and U.S. Bank's involvement.

Nobody was providing answers on Monday about how the protesters easily perforated security at multiple points. The protesters themselves planned a news conference Tuesday afternoon outside the stadium. According to his father, Mayo is experienced at protesting in high places and once spent nearly a year in a tree in Oregon.

Zimmerman said he's proud of his son for bringing attention to global warming and fossil fuel dependence. "Who wouldn't be?" he said. As to how his son got into the stadium with his equipment and sign, Zimmerman said, "I have no idea."

SMG, the firm that operates the stadium, issued a written statement from marketing manager Lisa Niess that said in part, "I want to reiterate that we are working with the Minneapolis Police Department and all stadium partners to fully investigate the situation."

The Vikings released a statement saying the team is "deeply concerned" about the incident and is awaiting the results of the police probe as well as the investigation led by SMG, the Minnesota Sports Facilities Authority (MSFA) and Monterrey Security, the firm that provides protection for the stadium.

"At that time we will look at the situation comprehensively so that all measures have been taken to ensure such incidents do not happen again," the team said in the statement.

No one was harmed during the protest, but the prospect of a more alarming breach wasn't hard to conjure.

The protest began midway through the second quarter.

The climbers remained on high until the game ended, then they easily hoisted themselves to the catwalk.

Police and firefighters had climbed to the catwalk above the protesters and spoke with them at various times. They arrested them on the spot.

Chicago-based Monterrey Security has the contract to provide security for the building. Minneapolis police also are prominent in and around the stadium on game days.

Calls to Monterrey were not returned Monday.

Monterrey employees stand by metal detectors at entrances to the stadium. All employees and spectators must pass through metal detectors and open their bags for inspection upon entering the building that seats more than 66,000.

For Minnesota Vikings games, the NFL allows fans to carry small clutches — about the size of a hand — but bigger bags must be clear and no larger than 12 inches by 6 inches by 12 inches.

The rectangular banner was significantly larger than the limits. The sign carried the U.S. Bank logo and read, "Divest #NoDAPL," a reference to the Dakota Access Pipeline in North Dakota, which has been the subject of national controversy and a monthslong protest encampment.

Niess confirmed there were events in the building late Saturday night and that all guests would have gone through security.

Jenn Hathaway, the spokeswoman for MSFA, declined to comment on the incident.

About 185 seats below the protesters were evacuated during the game, displacing the fans who were moved into the Truss Bar, a luxury area on the upper concourse.

The Vikings also intend to refund the fans' ticket prices.

Twitter: @rochelleolson