SANT'AGOSTINO DI FERRARA, Italy (AP) — A magnitude-6.0 earthquake shook several small towns in northeast Italy yesterday, killing four people, knocking down a clock tower and other centuries-old buildings and causing millions in losses to the region known for making Parmesan cheese.

The quake struck at 4:04 am, with its epicenter about 22 miles north of Bologna at a relatively shallow depth of 3.2 miles, the US Geological Survey said. Civil protection agency official Adriano Gumina described it as the worst quake to hit the region since the 1300s.

The four people killed were factory workers on the overnight shift when their buildings, in three separate locations, collapsed, agency chief Franco Gabrielli said, In addition, he said, two women died — apparently of heart attacks that may have been sparked by fear. Sky TG24 TV reported one of them was about 100 years old.

Gabrielli said dozens of people were injured.

Two of the dead were workers at a ceramics factory in the town of Sant'Agostino di Ferrara. Their cavernous building turned into a pile of rubble, leaving twisted metal supports jutting out at odd angles and the roof mangled.

"This is immense damage, but the worst part is we lost two people," fellow worker Stefano Zeni said. News reports said one of the dead had worked the shift of an ill colleague. Elsewhere in the town, another worker was found dead under factory rubble.

In the town of Ponte Rodoni di Bondeno, a worker also died as his factory collapsed, emergency workers told Italian news agencies.

Premier Mario Monti, in Chicago for the NATO summit, told reporters he was returning to Italy before the meeting ends because of the quake.

The quake struck in the farm region known for production of Parmigiano and Grana cheeses. Italy's farm lobby Coldiretti said that some 200,000 huge, round cheeses were damaged, causing a loss to producers of (euro) 50 million ($65 milion).

It also said in a statement that at least three barn roofs collapsed, trapping an unspecified number of pigs and milk cows inside.