The morning after the killing of al-Qaeda leader Osama bin Laden was announced, cannons boomed in the Washington area, but it was neither as retaliation nor in triumph.

Many people were unsure of just what it was or why.

“It sounded like a large pile-driver, which started at approximately 0635 hours,” a D.C. police official wrote on a police Web site. “Unsure of the exact origin.”

“It sounds to me like trucks driving over a steel plate in the road,” a Capitol Hill area woman wrote on the site.

The steady thumps and thuds were produced at Arlington National Cemetery by an elite military ceremonial unit, the Presidential Salute Battery, practicing to maintain proficiency, according to a release from the military.

The firing had been announced the week before, and it was timed to avoid interfering with memorial services. Some who didn’t get the message seemed displeased.

The DCist blog said it received complaints. “Not good timing,” read one posting. “I admit I immediately flicked on the TV to see if it was ‘retribution.’ ”

“Same here,” said another.

Additional practice is scheduled Tuesday from 7 to 8 a.m.