The day-long scare began when a suspicious package was found on a steel beam under a bridge in the morning. Police stopped traffic on a major interstate highway north of Boston, cordoned off the area, deployed a bomb squad and blew it up.

By afternoon, at least eight other similar suspicious packages were discovered, each triggering a security alert involving emergency crews, federal agents, bomb squads, police and the U.S. Coast Guard.

"The 'packages' in question are magnetic lights that pose no danger," Turner Broadcasting System Inc., a unit of Time Warner Inc., said in statement.

It said the devices were part of an outdoor marketing campaign in 10 American cities for an animated television show "Aqua Teen Hunger Force" produced by its Adult Swim brand. They had been in place for two to three weeks in Boston, New York, Los Angeles, Chicago, Atlanta, Seattle, Portland, Austin, San Francisco, and Philadelphia.

"We regret that they were mistakenly thought to pose any danger," said Turner, adding that it had contacted law enforcement officials to give them the precise locations of the packages.

The discoveries in Boston triggered the biggest city-wide security alert since the September 11, 2001, attacks.

It forced the U.S. Coast Guard to close the Charles River that feeds from the Atlantic Ocean into the city and caused authorities to shut down major bridges linking Boston with neighboring Cambridge along with several roads.

"This has taken a significant toll on our resources," Boston Police Commissioner Edward Davis told reporters.