A flag flies Thursday morning outside the Milwaukee County War Memorial Center after a ceremony marking 13 years since the Sept. 11 terrorist attacks. Credit: Ashley Luthern

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Images of the Sept. 11 terror attacks are seared into American consciousness.

A falling tower, then another. A gaping hole in the Pentagon. A smoking plane in a field.

Gov. Scott Walker drew another image from his memory Thursday during a ceremony at the Milwaukee County War Memorial Center on the 13th anniversary of the attacks.

It was a barn with a smiley face painted on it. On Sept. 11, 2001, Walker was carpooling from Milwaukee to Madison, when, while on I-94 near the Delafield exit, a radio announcer said the twin towers were coming down.

He already knew about the planes. His wife, Tonette, had called that morning as he got their sons ready for Roosevelt Elementary School in Wauwatosa. She said to turn on the TV — the one in the kitchen, not the living room — and Walker saw a plane hit the first tower. Like many parents that day, he sent his children to class.

When he heard in the car that the towers had fallen, Walker said that his reactions ranged from disbelief to shock and fear. That Friday, Walker and his family invited their friends to answer the president's call to light candles on front porches around the country. Dozens of people stood in their lawn, he said, where they sang "God Bless America" and prayed.

"I hope you remember not just what happened on that day on September 11, but what happened on days like that Friday where, as a country and as freedom-loving people around the world, we stood united," he said.

"It didn't matter if you were a Democrat or a Republican. It didn't matter if you were black or white. It didn't matter if you were rich or poor, we stood together united," Walker continued. "We need to get back to times like that here in Wisconsin. We need to get back to times like that here in the United States. We need to get back to times like that all around the world."

Walker spoke to a diverse crowd of first responders, public safety officials, veterans, military members and politicians, including Mary Burke, Walker's opponent in the governor's race this fall.

Milwaukee Mayor Tom Barrett, County Executive Chris Abele and British Ambassador Sir Peter Westmacott also offered remarks. Westmacott recently made headlines when he said during a TV interview that police and spy agencies are close to identifying the English-accented militant depicted on video showing the killing of U.S. journalist James Foley. Foley, a Marquette University alumnus, was kidnapped in November 2012 while covering the Syrian civil war.

Islamic State extremists have claimed credit for the execution of Foley and another American journalist, Steven Sotloff. On Wednesday, President Barack Obama authorized U.S. airstrikes inside Syria, along with expanded strikes in Iraq as part of "a steady, relentless effort" to root out Islamic State extremists and their spreading reign of terror.

The president's announcement of the airstrikes serves as a reminder that "this is unfortunately still work in progress," Westmacott said.

"The determination and the commitment of the British government, the British people, to stand alongside the American government and the American people to defeat the scourge of mindless terrorism is undiminished," he said. "We will stand with you and we will not forget."