Four Jamaican women were arrested today on suspicion of taking part in sham marriages, the UK Border Agency said.
Police officers and immigration officials carried out dawn raids on 10 addresses in Birmingham and one in Wolverhampton.
The four women, who were all arrested in Birmingham, are alleged to have married British citizens solely so they could remain in the UK after overstaying their visas.
The raids were carried out by three teams made up of a total of 18 officers from West Midlands Police and the UK Border Agency.
Officials are continuing to search for another seven people who were not at home at the time of the raids.
A UK Border Agency spokesman said: "All four women are believed to have tried to legitimise their stay in the UK by marrying a British citizen.
"All four originally came on visas which they overstayed."
The raids were based largely on intelligence provided by Venrick Clarke, who was jailed last year for masterminding a sham marriage scheme.