A homicide bomber struck police lined up at the entrance of the main police academy in Baghdad on Sunday, killing 28 people and wounding 57, sources tell FOX News.

The blast — the second major attack to hit Iraqis in three days — was a bloody reminder of the ability of insurgents to defy security improvements and stage dramatic attacks as the U.S. begins to draw down its forces. The U.S. military was expected to announce the withdrawal of two more brigades on Sunday.

The bomber detonated his explosives near a side entrance of the academy, which is in a mainly Shiite area of eastern Baghdad.

One police officer said the bomber was riding a motorcycle when he drove into the line, but the Interior Ministry said the method of attack had not yet been determined.

Extremists have increasingly targeted Iraqi forces as they try to prove they can take over the country's security so the American troops can go home.

Violence has declined sharply in Baghdad and surrounding areas, but Iraqis continue to face attacks daily.

Baghdad's main police academy has been hit by several bombings. Another homicide bombing there killed at least 33 people and wounded dozens on Dec. 1.

Iraqi officials provided conflicting casualty tolls, as is common in the chaotic aftermath of bombings.

Two medical officials and one police officer in the area where the bombing occurred said 30 people were killed and some 60 wounded.

The officials spoke on condition of anonymity because they weren't authorized to release the information.

Interior Ministry spokesman Maj. Gen. Abdul-Karim Khalaf said 24 people were killed and more than 60 wounded.

Nobody claimed responsibility for Sunday's attack. But homicide attacks and car bombings are usually blamed on Al Qaeda in Iraq and other Sunni insurgents seeking to undermine public confidence in security gains that have led to a sharp decline in violence.

Sunday's bombing was the deadliest to strike Baghdad in nearly a month.

Bombs struck Shiite pilgrims heading to religious ceremonies in a western section of the capital on Feb. 11, killing at least 16 people.

But a spate of deadly attacks have occurred in other areas.

On Thursday, a parked car bomb tore through a livestock market in Hillah, 60 miles south of Baghdad, killing 13 people.

The Associated Press contributed to this report.