Baghdad, Iraq ? A roadside bomb destroyed a second heavily armored Bradley Fighting Vehicle in less than a week Monday, killing two U.S. soldiers, wounding four others and indicating that insurgents have increased the power of the explosives they are using against American troops.

The blast came hours after gunmen in a passing car assassinated Baghdad’s deputy police chief and his son while they drove to work, part of a campaign to target Iraq’s security forces. Al-Qaida in Iraq, the group led by Jordanian terrorist Abu Musab al-Zarqawi, claimed responsibility.

American officials have cautioned that insurgents will escalate attacks in a bid to scuttle Jan. 30 elections. After a roadside bomb struck a Bradley on Thursday and killed seven soldiers, the Defense Department warned that militants were increasing the size and power of their bombs.

The attack Monday on a Bradley in southwest Baghdad followed the same pattern.

“It’s fair to say that they are afraid of the elections, they are afraid of what the outcome will be, and they want to do everything they can to derail that process because that’s just one more step toward their demise,” U.S. military spokesman Lt. Col. James Hutton said. “This is probably an indication of their increase in effort and investment to derail the vote.”

The attack was one of several acts of major violence Monday.

A suicide attacker detonated a bomb in a fake police car at a police station courtyard in Baghdad, killing at least four officers and wounding 10 during a shift change, police and witnesses said.

On Monday, attackers shot and killed Baghdad’s deputy police chief, Brig. Amer Ali Nayef, and his son, Lt. Khalid Amer, also a police officer. They were slain in Baghdad’s Dora district while traveling in a car on their way to work, Interior Ministry spokesman Capt. Ahmed Ismail said.

Al-Qaida in Iraq claimed responsibility for the Baghdad assassinations in a statement posted on the Internet, describing Nayef and Amer as foreign “agents.” The statement warned that other Iraqis cooperating with the U.S.-led military would meet the same fate.