LONDON, Aug. 5 — A recent breakthrough in the investigation of the killing of a Nigerian boy found headless and limbless in the Thames River almost two years ago has brought new scrutiny to the growth of child trafficking in Britain.

Last week, riot police officers arrested 21 people here in a series of raids as part of the investigation into the death of the boy they call Adam. The child, who was between 4 and 7, is believed to have been the victim of a ritualistic sacrifice after being smuggled from Nigeria.

"We are pretty confident that we have a group of individuals who would have trafficked Adam into the country, but our inquiries are still at a very, very early stage," Detective Inspector Will O'Reilly said at a news conference after the raids.

Inspector O'Reilly added that the police had uncovered what they believe is a "criminal network concentrating on people trafficking, particularly from mainland Africa through Europe to the U.K."

Among the evidence seized by the police was an animal skull with a nail through its head and an assortment of soil and clay. "These items obviously have some ritualistic meaning and we are bringing them back to our experts," Inspector O'Reilly said.

Law enforcement officials have long suspected that Adam fell victim to a child trafficking ring.

There has been an increase reported in child trafficking in Britain, both as a destination and as a transit point, reflecting a worldwide increase in the crime. The children have turned up in an array of cities, including places like Newcastle, a long way from Britain's main airports and seaports.

There are also signs that more and more children are coming from African countries, including Nigeria — where Adam came from — Sierra Leone, Angola and Burundi.

A newly published report by the British branch of Unicef highlighted the growth of the problem in Britain, where it said there were hundreds of known cases of children being forced into sexual and domestic slavery. Since the cases are difficult to detect, the more accurate number is almost certainly in the thousands, said the report, titled "Stop the Traffic."

"It's very hard to see how the authorities can be picking up more than a small fraction of the actual cases that are arriving here," said David Bull, executive director of Unicef U.K. "The chances are quite strong that a trafficked child arriving here under current arrangements won't be identified and protected."

In Adam's case, the police faced almost insurmountable odds in gathering evidence. When they found the boy's body, clothed in only a pair of girl's orange gym shorts, floating in the river in September 2001, they did not know who he was or where he was killed.

Using forensic investigative tools, Scotland Yard chipped away at the case. Pollen found in the boy's body meant he was alive when he arrived in London. Tests on the mineral levels in his bones established that he came from within 100 miles of the Benin City area in Nigeria.

A break in the case came a year ago when the police arrested, and later released, a Nigerian woman who was reported to have told social services in Scotland that she wanted to perform a ritual with her children.

The police searched her house and found a pair of orange gym shorts, the same brand and size as Adam's, which are available only in Germany. Her arrest led to other arrests.

Earlier this month, Sam Onojhighovie, 37, a Nigerian man who the police said could be the boy's father, was arrested. He faces extradition to Germany, where he has already been convicted and sentenced for crimes linked to trafficking in people.

Those arrested last week will have DNA tests to see if they are related to Adam.

The British government is in the process of strengthening an existing law that makes it illegal to import a child for sexual purposes. But the bill would do little to close a loophole that permits child trafficking for other reasons, like domestic servitude.

At the same time, Britain is getting ready to close its only safe house for children who have been the victims of child trafficking, a development that worries children's advocates.

"This should be seen as a child protection issue, not an immigration issue," Mr. Bull said.