(CNN) The chairman of the Senate Armed Services Committee is urging President Donald Trump to quickly nominate a permanent secretary of defense.
Sen. James Inhofe, an Oklahoma Republican, warned on Tuesday that "we have too many threats facing this nation to have so many key DOD officials serving in an 'acting' capacity."
Acting Defense Secretary Patrick Shanahan, who took up the role in January following James Mattis' resignation, has been told by the Trump administration that his possible nomination for the permanent role is in a "holding pattern" after the Pentagon inspector general launched an investigation into possible ethics violations last week, according to a source close to Shanahan.
Inhofe told CNN on Tuesday that if the White House chooses "to nominate acting Secretary of Defense Shanahan," he would "welcome the DODIG's (Pentagon inspector general's) thorough, but expeditious examination of any outstanding issues."
A senior administration official said the White House isn't likely to move on nominating Shanahan while the investigation is underway. Administration officials tell CNN they are unsure how long the inspector general's investigation may take.
Nonetheless, several sources close to Shanahan are privately mounting an all-out defense of his nomination, saying there is no evidence he violated his ethics agreement and that he is convinced the inspector general will find no evidence of wrongdoing.
Army Secretary Mark Esper's name has surfaced again as a possible contender, according to multiple officials, but it is unclear if Trump will wait for the final report or nominate another candidate. Some Army officials believe Esper is not interested in the job, and he has refused to comment.
The inspector general's investigation was opened after a complaint from a public interest group, Citizens for Responsibility and Ethics in Washington, that Shanahan, a former high-level executive at Boeing, allegedly violated ethics rules by criticizing Lockheed Martin -- Boeing's competitor on military fighter jet contracts. His ethics agreement mandates that he not engage in any action that could benefit Boeing.
Shanahan has said he welcomes the investigation.
The official close to Shanahan says the acting secretary has never done or said anything that would directly benefit Boeing, but acknowledged he has implicitly criticized Lockheed Martin's F-35 fighter program by talking positively about how he managed some Boeing commercial aircraft projects, including the Dreamliner, when he was at the company. Administration officials say the inspector general is examining multiple reports of potential violations.
Beyond the top job, there is uncertainty about a number of key positions at the Pentagon. If Shanahan is confirmed, or decides to leave, it would mean a new deputy secretary would have to be confirmed.
In addition, Gen. Mark Milley, chief of staff of the Army, has yet to be formally nominated to become chairman of the joint chiefs of staff, and there is no nominee for vice chairman of the joint chiefs, a job that will come open in the next few months.
Trump also has to fill the job of Air Force secretary, with Heather Wilson scheduled to leave office in May.
Further, the Marine Corps is waiting for a new nominee to become commandant. It had been widely expected for days, and no one is certain what is holding up the White House.