PHILADELPHIA (Reuters) - A local newspaper in northwest Pennsylvania apologized on Friday for running a classified advertisement that called implicitly for the assassination of U.S. President Barack Obama.

President Barack Obama walks past F-16 aircraft at Nellis Air Force Base in Las Vegas, Nevada May 27, 2009. REUTERS/Jason Reed

The Warren Times Observer, with a circulation of about 11,000, ran the ad in Thursday’s paper and pulled it as soon as it was discovered by a manager, said publisher John Elchert.

The ad read: “May Obama follow in the footsteps of Lincoln, Garfield, McKinley and Kennedy,” Elchert said.

Elchert declined to identify the person who placed the ad, and said he had referred the case to local police, who notified the Secret Service.

Elchert said the representative who took the ad apparently didn’t make the connection between the four assassinated presidents -- Abraham Lincoln, James Garfield, William McKinley and John F. Kennedy -- and mistakenly allowed the ad to run.

“It was just an honest mistake,” he told Reuters.

Jim Mackin, a spokesman for the Secret Service, said investigators had identified the person who placed the ad and were taking it seriously. “We do not have the luxury of doing otherwise,” he said.

Concerns about Obama’s safety arose early in his campaign for president, especially among African Americans who expressed fears he could meet the same fate as black civil rights leader Martin Luther King, assassinated in 1968.

The Secret Service provided round-the-clock protection for Obama, the first black president in U.S. history, from an early stage in the campaign.

In a post on its website on Friday, the newspaper said the ad “apparently alludes to the wish that President Obama meets an untimely end by linking him with four assassinated presidents.” It said the paper apologized for the “oversight.”

In the 2008 general election, voters in the surrounding Warren County, about 300 miles northwest of Philadelphia, backed Republican candidate John McCain with 52 percent of the vote against Democrat Barack Obama with 46 percent.