North Yorkshire's chief constable and his deputy are being investigated over allegations they helped relatives during a police recruitment exercise.
The Independent Police Complaints Commission said the claims involve Chief Constable Grahame Maxwell, deputy Adam Briggs and two other police staff.
A jobs drive was held last month where 200,000 people called about 60 posts.
The force said its main concern was to reassure people its "recruitment process is fair and transparent".
The Independent Police Complaints Commission (IPCC) said the police officer recruitment exercise, which ran from 8 to 12 February, "encountered technical difficulties on the first two nights with consequent frustrations for many potential applicants".
The IPCC said: "It is alleged the Chief Constable, Grahame Maxwell, gave assistance to one of his own relatives and a relative of Deputy Chief Constable Adam Briggs.
"It is also alleged DCC Briggs gave assistance to that same relative.
"Additionally, and separately, two police staff members are alleged to have used their positions to circumvent the initial phase to benefit themselves and one other person."
The commission said an investigation into the claims against the two senior officers was being conducted by Chief Constable Mike Cunningham from Staffordshire Police.
The inquiry relating to the other two members of staff, who have not been named, will be be carried out by North Yorkshire Police's professional standards department.
North Yorkshire Assistant Chief Constable Sue Cross said: "In respect of the IPCC investigations, our principal concern is to reassure applicants and the communities we serve that our recruitment process is fair and transparent.
"To that end, North Yorkshire Police is currently conducting a very thorough integrity assurance exercise."
Ms Cross said the force was co-operating fully with the IPCC investigation.