Labour has formally written to the head of the civil service to seek more information about what it called “incredibly alarming revelations” that Liz Truss’s chief of staff, Mark Fullbrook, was questioned as a witness as part of an FBI inquiry. Angela Rayner, Labour’s deputy leader, asked to know when Simon Case, the cabinet secretary, first knew about the information. She also asked when Truss was informed, and whether Fullbrook had revealed his link to the investigation before taking the job. Rayner said she was writing to Case because Truss still lacked an independent adviser on ministers’ interests after the last incumbent, Christopher Geidt, resigned in June. Boris Johnson did not replace him, and Truss has indicated that she might get rid of the ethics adviser role. Truss has given her full support to Fullbrook after it emerged that he was questioned as a witness as part of an FBI inquiry into alleged bribery in Puerto Rico. She declined to say whether she was told about his cooperation with the FBI before his appointment, saying only that it had gone through the “proper process”. In her letter to Case, Rayner said Fullbrook’s links to the matter, first reported by the Sunday Times, were “incredibly alarming revelations which the public will rightly want clarity on”. The claims “once again prompt questions about this government’s ethics, values and basic standards of decency”, Rayner wrote. “In the interest of transparency and out of respect for our democratic institutions, I urge you to clarify when you were first made aware of these allegations. Importantly, at what stage was this knowledge shared with the prime minister? Were any declarations made by Mr Fullbrook about his involvement in this investigation when he took on the position of chief of staff?” The FBI investigation relates to allegations that Julio Herrera Velutini, a financier and Tory donor, promised to help the former governor of Puerto Rico to get re-elected if she dismissed an official investigating a bank he owned there. He has denied the charges against him. A spokesperson for Fullbrook said: “As has been made repeatedly clear, Mr Fullbrook is committed to and complies with all laws and regulations in any jurisdiction in which he works and is confident that he has done so in this matter. “Indeed, Mark Fullbrook is a witness in this matter and has fully, completely and voluntarily engaged with the US authorities in this matter, as he would always do in any circumstance in which his assistance is sought by authorities. “The work was engaged only by Mr Herrera and only to conduct opinion research for him and no one else. “Mr Fullbrook never did any work for, nor presented any research findings to, the governor or her campaign. There has been no engagement since. Mr Fullbrook understands that there are active legal proceedings against other individuals and entities. It would therefore be inappropriate to comment further.”