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Mr. Donovan had said the truck had equipment but no weapons on board. He declined to say whether any of the equipment was stolen.

Messages seeking information were left for Detroit police on Monday.

Mr. Biden addressed a union rally Monday to mark Labor Day. The rally site is near the hotel.

NORTH CAROLINA

New Obama ad makes appeal to middle class

CHARLOTTE — President Obama’s campaign is running a new television ad claiming Republican Mitt Romney’s policies would “hit the middle class harder.”

The ad says Mr. Romney wants to extend tax cuts for millionaires at the expense of middle-income earners.

Mr. Romney has called for an extension of tax cuts first approved by George W. Bush for all income earners. Mr. Obama wants to let the cuts expire at the end of the year for families making more than $250,000 a year.

The ad says Mr. Romney doesn’t see the “heavy load” the middle class is carrying.

The new spot is airing as Democrats open their party convention in Charlotte, where they’ll focus on Mr. Obama’s economic plans for the middle class.

The ad is running in Colorado, Iowa, New Hampshire, Nevada, Ohio and Virginia, key battleground states.

Responding to the ad, Romney campaign spokeswoman Amanda Henneberg said Mr. Obama is the only candidate who wants to raise taxes, while Mr. Romney has a plan to lower rates, help create jobs and boost take-home pay.

• From wire dispatches and staff reports