Suicide bombers hit a bus carrying intelligence agency employees and a checkpoint near the headquarters of the Pakistan army on Saturday, killing at least 35 people, an official said. The two attackers struck in Rawalpindi, a garrison city just south of the capital, Islamabad. A senior intelligence official, who asked for anonymity because of the sensitivity of his work, said at least 35 people were killed. In the first attack, an explosive laden car rammed a bus carrying employees from the Inter-Services Intelligence agency, while the other bomber hit the checkpoint, said Mohammed Afzal, a local police official. An intelligence agent at the scene said the destroyed bus was a 72-seater, but that more people were on board. The agent, who asked not to be identified, said he believed casualty numbers would keep rising. After the blast, troops and police quickly blocked off the area of the blast. They pushed back bystanders and snatched cameras and mobile phones from journalists and bystanders. Agents fanned out across the area, picking up metal bits of what appeared to be the suicide bomber's car. It was the second major attack against the ISI in recent months. On September 4, a suicide attacker blew himself up after boarding a bus carrying ISI's employees, while a roadside bomb went off near a commercial area in Rawalpindi minutes apart, killing at least 25 people. Islamic militants have launched dozens of suicide attacks this year. Most have taken place near the Afghan border, but several have taken place in the country's main cities, raising fears that violent extremism is spreading. A bomber blew himself up in Rawalpindi on October 30 at a checkpoint several hundred meters from the office of President Gen. Pervez Musharraf, killing seven people. Two weeks earlier, a suicide attack on opposition leader Benazir Bhutto's homecoming parade killed more than 140 people in Pakistan's southern port city of Karachi. Authorities cited the risk of further attacks when they barred Bhutto from holding a rally in Rawalpindi last month against Musharraf's imposition of emergency rule. Tension between militant groups and the military are at a high because of an ongoing military operation to sweep the followers of a pro-Taliban cleric from the northern Swat valley.