BAGHDAD, Dec. 29 -- Insurgents lured police to a house in western Baghdad and then set off a powerful explosion that killed at least 28 people late Tuesday night, including three families whose homes were completely flattened in the blast, officials said Wednesday.
The debris was still smoldering as dawn broke, showing the intensity of the damage, which the U.S. military estimated came from 1,700 to 1,800 pounds of explosives.

A crowd gathers around the site of Tuesday night's explosion in Baghdad. Officials said insurgents lured police officers by firing shots from the house.
(Hadi Mizban -- AP)
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The attack was unusual for a capital city accustomed to roadside bombs, suicide car bombs, tractor bombs, donkey bombs and cart bombs. This appeared to be a house bomb -- the first of its kind -- and the enormous amount of explosives used in the attack pointed to a sophisticated plot. U.S. and Iraqi security forces said an investigation was underway.
Col. Adnan Abdul Rahman, a spokesman for the Iraqi Interior Ministry, said neighbors called police to the house after a man began shooting from the roof.
"When the police came, the house blew up," Rahman said. "It was an ambush. He meant to kill the people and the National Guard and police."
The blast killed six police officers and 22 civilians, according to the Iraqi government. Four police officers were still missing Wednesday as a crane operator picked through the rubble in the Ghazaliya neighborhood. Twenty-one people were injured in the explosion.
Meanwhile, in the northern city of Mosul, 15 American soldiers were wounded in a large, coordinated attack on a U.S. outpost in the western part of the city, the military said. Nine of the soldiers were treated and returned to duty, said Lt. Col. Paul Hastings, spokesman for Task Force Olympia in Mosul.
Hastings said a car bomb hit the Army outpost near Yarmouk traffic circle at about 3:45 p.m. Civilian witnesses described the outpost as an abandoned apartment building on Baghdad Street that soldiers had seized and begun occupying two weeks ago.
A patrol responding to that attack was hit by another car bomb and a roadside bomb, Hastings said. About 50 insurgents tried to overrun the building with gunfire and rocket-propelled grenades. At least half of them were killed, Hastings said.
Witnesses said one of the car bombs was actually carried by a fuel tanker, which took down part of the four-story building when it exploded. Two U.S. tanks that had been parked in front were destroyed -- one buried in the rubble, the other burned, witnesses said.
"Half of it was collapsed, and heavy smoke was coming out," said Monther Jasim, 28, who lived in the neighborhood. "I was frightened to get too close."
U.S. warplanes roared overhead following the attacks, witnesses said.
"Every day I go by the building, and usually there are two armored vehicles outside," said one resident, Sabah Salim, 30. "After the explosion, I didn't see the armored vehicles, only parts, because the bricks and parts of the building were covering one and the other was destroyed from the car bomb."
Witnesses said U.S. soldiers had been conducting operations in the area for the past couple of weeks. Mosul, 220 miles north of Baghdad, has become a trouble spot for security forces and civilians, who have felt the deadly sting of a series of insurgent attacks in the past two months. A suicide bomber penetrated security at Forward Operating Base Marez on Dec. 21, detonating explosives that killed 22 people, including 14 U.S. service members. The attack, which also wounded 69 people, was the deadliest on an American installation since the beginning of the war in March 2003.