London ? An aide to Prime Minister Tony Blair was arrested Friday in the investigation into whether political honors, such as seats in the House of Lords and knighthoods, were traded for cash.

Ruth Turner, Blair’s director of government relations, was questioned on suspicion of being involved in the sale of honors – illegal under British law – and perverting the course of justice.

Turner, 36, is the first member of Blair’s Downing Street staff to be arrested and the fourth person to be detained by police in their investigation. All four are on bail while inquiries continue, police said.

Blair was interviewed by detectives last month, becoming the first serving prime minister interviewed in a criminal inquiry. However, he was not interviewed “under caution,” meaning it is unlikely he is a suspect.