Liz Truss has given her full support to the No 10 chief of staff, Mark Fullbrook, after it emerged he was questioned as a witness as part of a Federal Bureau of Investigation (FBI) inquiry into alleged bribery in Puerto Rico. Truss’s spokesperson said she was backing Fullbrook, who is not travelling with her to the UN general assembly in New York this week. But the prime minister herself declined to say whether she had been told about his cooperation with the FBI before his appointment, saying only that it had gone through the “proper process”. No 10 said Fullbrook’s decision not to travel with the prime minister was because he was working on government business, including the energy package for businesses on Wednesday and the mini-budget on Friday. A Downing Street spokesperson said the prime minister stood 100% behind Fullbrook and he had “her full support”. The FBI investigation relates to allegations that Julio Herrera Velutini, a financier and Tory donor, promised to help the former governor of Puerto Rico 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.”