Suspected al-Qaida bombers stepped up attacks on key transportation arteries, striking a bridge north of the capital a day after shutting the superhighway south of Baghdad with a huge explosion that collapsed an overpass and killed three US soldiers. The latest attack, a parked truck bomb, blew apart the bridge that carries traffic over the Diyala River in Baqouba, police said on condition they not be identified by name because they feared retribution. There were no casualties, but motorists and truckers now must use a road that runs through al-Qaida-controlled territory to reach important nearby cities. Baqouba is the capital of Diyala province, which is swarming with al-Qaida fighters. Those terrorists were driven out of Baghdad by the four-month-old US security operation and out of Anbar province west of the capital by Sunni tribesman who rose up against the terrorist group. The attacks on the bridges were only the latest in a campaign to deepen turmoil in Iraq, especially on the vital transportation network linking Baghdad to the rest of the country. Such bombings, especially suicide attacks, are an al-Qaida trademark and one of the group's many and ever-shifting tactics against US and Iraqi forces. Earlier this month, a bomb heavily damaged the Sarhat Bridge, a key crossing 90 miles (145 kilometers) north of the capital on a major road connecting Baghdad with Irbil, Sulaimaniya and other Kurdish cities.