When the U.S. led an air campaign over Kosovo more than a decade ago, the Pentagon summoned a squadron of radar-jamming Prowler jets to help keep the skies safe for American bombers.

Lives depended on the unit, and the unit depended on its intelligence officer — Lt. Cmdr. Mark Kirk, who is now the Republican candidate to be Illinois' next senator.

Since launching his campaign, Kirk has come under sharp criticism for exaggerating his military accomplishments, repeatedly choosing to emphasize the few "Top Gun" moments in a 21-year Reserve career that has been spent almost entirely focused on office work.

But an Associated Press review of Navy personnel documents — as well as interviews with former colleagues, commanders and experts — shows Kirk has been an exceptional officer entrusted with vital, sensitive duties. His work was important but not glamorous.

If Kirk had limited his statements to his actual military record, he would not have lacked for achievements to brag about.

During the Kosovo bombing campaign, for instance, Kirk stayed behind at a base in Italy to study data and prepare briefings while the pilots of his unit, known as the "Star Warriors," were in the air.

But instead of describing his actual duties when talking to voters, Kirk has talked about encountering anti-aircraft fire over Kosovo on what he called his "first mission." But it was his only flight, and he was just an observer.

In some cases, Kirk exaggerated to the point of making false claims. He took sole credit for an award that went to his entire unit. He has sometimes claimed he served in the first Gulf War and the invasion of Iraq, even though he was in the United States for both conflicts. His campaign has used pictures showing Kirk in full combat gear or wearing a flight helmet in a cockpit.

The pattern of falsehoods now threatens Kirk's candidacy. He was forced to apologize for the misstatements. His opponent, Democrat Alexi Giannoulias, has branded Kirk a liar.

In support of Kirk's abilities, one superior gave him the "highest possible recommendation." Others called him a "go-to guy" and cited his "unlimited potential."

Kirk joined the Navy Reserve in 1989, when he was a young congressional aide. He stayed with it as he held increasingly important jobs with the State Department, the World Bank and the House International Relations Committee. Kirk continued serving even after being elected to Congress in 2000.

He has spent his entire career in military intelligence. Early on, that meant studying Latin America, particularly drug activity there. Later Kirk was assigned to the Joint Intelligence Center at the Pentagon, where he briefed top military leaders on crises.

As he rose in rank, Kirk eventually began taking shifts overseeing the entire intelligence section of the Pentagon war room, monitoring activity around the world in case anything required U.S. action.

He also served, temporarily, aboard ships such as the aircraft carrier John C. Stennis and in Afghanistan, where he again studied drug trafficking.

The fact that Kirk was entrusted with those positions suggests his superiors considered him one of the best, said Thom Kroon, a former Reserve intelligence analyst.

"They're going to be looking for a hard-charger who has done a great job for people in the past," Kroon said.

Military records indicate Kirk delivered a "brilliant and knowledgeable briefing on international economics." He established an intelligence team to monitor sudden developments in Cuba and Haiti. He prepared a map of combat forces in the Balkans. He was praised as "the finest orator" in the Navy.

"He was generally thought of, as I recall, as a very capable up-and-comer in a group of very capable up-and-comers," said Peter Feaver, who served in a Reserve intelligence unit with Kirk and is now a professor and national security expert at Duke University.

Kirk's time with the Prowler unit produced some of the most dramatic, high-stakes work of his career.

First during the Kosovo bombing campaign and then during Operation Northern Watch over Iraq, Kirk helped figure out where to send the EA-6B Prowler planes, what their goals should be and how to get them back home.

An intelligence officer's work is "pretty crucial" to the mission, said Sergio Posadas, himself a former intelligence officer and Prowler crew member in the Marines.

Experts say Kirk's work would have involved reviewing information from American intelligence agencies and pulling out the portions that would help Prowler pilots find the enemy. What frequencies had they been using? Where had they been located recently? Was there any trend to suggest where they would go next?

Michael Graham, who served in a stateside intelligence unit with Kirk for about five years, said Kirk figured out a way to uncover important information during the Kosovo conflict. A video of Kirk's presentation on the method was used in intelligence training for years afterward, he and others said.

During the Kosovo bombing campaign, Kirk coordinated intelligence for multiple squadrons — the largest Prowler intelligence group "in the history of naval aviation," according to a commendation that Kirk earned.

Kirk's group earned an award as top intelligence unit. He later began saying that he had been named the Navy's intelligence officer of the year.

Kirk's commanding officer at the time, Capt. F. Clay Fearnow, defends Kirk's incorrect claim. "There would have been no team without Mark Kirk's leadership, and there certainly would have been no award," Fearnow said in a statement distributed by Kirk's campaign.

Fearnow did not return calls for comment from The Associated Press.

Kirk would not agree to an interview and has largely stopped answering questions about his military service. He refers reporters to his fitness reports, which offer an overview of his work and the opinions of his commanding officers but do not include details of specific incidents. He refuses to release other records that would provide additional information about his service.

Kirk was not a pilot, but he took SERE training — for "survival, evasion, resistance and escape" — so that he could go along on some flights as an observer. Veterans and military experts said that would have provided valuable real-world experience for an intelligence officer.

Kirk's campaign did not say exactly how many times he flew but said records indicate it was at least three times — once over Kosovo but not in a Prowler, and more than once over Iraq.

Kirk and his campaign have described him coming under fire on those flights. But once questions about his military service arose, Kirk backtracked and said he could not be certain that his plane was specifically targeted on those flights.

Experts said they see no reason to doubt his account.

"If he flew in Kosovo, and he flew in Operation Northern Watch, I would be surprised if he weren't shot at," said Doug Roulstone, Kirk's commanding officer when he served aboard the John C. Stennis.

As an observer, Kirk would have been in a noisy jet listening to crew discussion of enemy fire and perhaps seeing tracers, said Posadas, the former Prowler crew member. It would have been easy to know there was activity without knowing whether the plane was in danger.

"It may feel very personal but never come close to your particular aircraft," Posadas said.

More recently, Kirk has made two trips to Afghanistan, spending two or three weeks there each time. His commanding officer on the first trip said Kirk showed "tremendous personal courage" while serving in dangerous areas.

Kirk has said that on his second trip, at the end of 2009, he was shot at. He will no longer answer questions about what happened.

Maj. Fred Tanner worked alongside Kirk on that first stint in Afghanistan. He said Kirk studied drug activity and, because of his past intelligence work involving drugs, was able to write a report with concrete suggestions for organizing the military's response.

Tanner said he was not expecting much from a member of Congress but Kirk threw himself into his work.

"Mark actually produced. He actually wrote stuff," Tanner said. "It kind of shocked me."