The relatives of two Pakistani men living in the UK expressed concerns that the pair are among 11 people being held in connection with an alleged al Qaida plot.
Twelve men - 11 Pakistani nationals and a UK-born Briton - were arrested after officers raided properties in Greater Manchester, Liverpool and Lancashire on Wednesday.
Armed police swooped on at least 14 addresses, including Liverpool's John Moores University, during six hours of frantic activity.
Officers have now been granted a further week to detain 11 of the men, who range in age from 22 to 41 and are being held in various locations across the country.
An 18-year-old man was released into the custody of the UK Border Agency, police said on Saturday.
Relatives of Abdul Wahab Khan and Mohammad Ramzan said they believe the two men are among the 11 suspects still being held.
The families, who live in the town of Dera Ismail Khan in north west Pakistan, said the two lived together and studied at John Moores University.
The relatives said they have been unable to reach the pair since the swoops were reported, and claimed they had learned of the young men's arrests through their friends.
Both families said no government officials from either country had contacted them. Abdul Wahab Khan's name was mentioned in several newspaper reports as one of the men alleged to have been arrested during Wednesday's raids.
His older brother, Gulzar Jan, said Khan came to Britain in 2006 and was studying for a master's degree in information technology. He said Khan, in his mid-20s, was not involved in politics and spent most of his energy on his studies.
