Baghdad, Iraq ? U.S. soldiers sent to beef up security in Baghdad were seen for the first time on the streets of the capital Saturday as Iraqi police used loudspeakers to reassure people that the Americans were there to protect them.

But at least 21 people were killed or found dead, most of them in the capital, which is being wracked by bombings and sectarian slayings. The dead included a Shiite couple and their two daughters abducted earlier in the day in Baghdad’s mostly Sunni area of Dora, police said.

With Sunni-Shiite killings on the rise, about 3,700 soldiers of the Army’s 172nd Stryker Brigade were brought from northern Iraq to bolster U.S. and Iraqi security forces that have struggled to contain the violence in Baghdad.

Several Stryker armored vehicles took positions in the mostly Sunni district of Ghazaliyah, one of the city’s most dangerous areas. Police used loudspeakers to encourage residents to reopen shops and go about their business normally because the soldiers would protect them.

U.S. commanders hope the presence of heavily armed American troops will intimidate sectarian death squads believed behind many of the killings and reassure Iraqis – especially Sunni Arabs – that they will be protected by Iraq’s predominantly Shiite security forces.

Moving the Stryker brigade to Baghdad, however, meant drawing down – at least temporarily – the U.S. military presence in northern Iraq. The brigade was based in Mosul but had subordinate units scattered over a wide area, including routes used by foreign militants slipping in from Syria.

Fears of trouble in the north rose Friday when a car bomb killed a police colonel in Mosul, triggering a firefight between police and insurgents. Iraq’s Defense Ministry and police announced that 55 suspected insurgents had been captured around Mosul after the violence.

A curfew remained in effect for a second day in the eastern part of Mosul while police searched for insurgents who escaped.

The U.S. command believed the risk of moving the Stryker brigade was worth taking because of the grave situation in Baghdad, where sectarian tensions are high. Sectarian bloodshed has soared in the capital since the Feb. 22 bombing of a Shiite shrine in Samarra, which triggered reprisal attacks on Sunnis.