By Barbara Starr and Zachary Cohen, CNN

(CNN) -- The Department of Defense's Inspector General announced Thursday that it has cleared acting Defense Secretary Patrick Shanahan of allegations he violated ethics agreements by promoting the interests of his former employer, Boeing.

The Inspector General "did not substantiate any of the allegations and determined that Acting Secretary Shanahan fully complied with his ethical obligations and agreements regarding Boeing and its competitors," according to a statement from the Pentagon. More than 30 witnesses, including Shanahan, were interviewed by the IG's office, according to the statement.

"The Office of Inspector General took these allegations seriously, and our 43-page report of investigation, which we released today, describes our conclusions and the facts on which they are based. The evidence showed that Acting Secretary Shanahan fully complied with his ethical obligations and ethical agreements with regard to Boeing and its competitors," Glenn Fine, the IG official who oversaw the investigation, said in an additional statement.

CNN has previously reported that if Shanahan is cleared of the allegations, President Donald Trump may decide to move ahead and send his nomination to the Senate to become the permanent defense chief.

So far, there are no indications that the White House is considering candidates other than Shanahan. Officials close to the acting secretary say he strongly believes he will be nominated by the White House.

The White House and Pentagon have both indicated Shanahan could not be nominated until the IG investigation concluded.

Shanahan, who worked at Boeing for more than 30 years prior to taking a job at the Pentagon, faced accusations of being overly warm toward the company and claims that he "disparaged the company's competitors to his subordinates" from a federal watchdog called Citizens for Responsibility and Ethics.

CREW filed a complaint in March asking the inspector general to launch a probe into whether Shanahan's actions violated his signed ethics agreement which aims to ensure there are no conflicts of interest with Boeing while he is at the Pentagon.

"The Department of Defense Office of Inspector General has decided to investigate complaints that we recently received that Acting Secretary Shanahan allegedly took actions to promote his former employer, Boeing, and disparage its competitors, allegedly in violation of ethics rules," a spokesman for the office responded in a statement when the probe was launched.

A US defense official told CNN at the time that the IG chose to launch the investigation because there is enough credible initial information beyond the complaint filed by CREW to warrant a probe into whether Shanahan violated his ethics agreement.

Shanahan's predecessor James Mattis and Chairman of the Joint Chiefs of Staff Joseph Dunford were among those interviewed for the report which offered new details about additional allegations that were referred to the Inspector General.

"On February 7, 2019, a Senate Committee on Armed Services attorney forwarded an anonymous allegation to the DoD OIG that asserted Mr. Shanahan violated ethics rules by promoting Boeing," the report states.

"For example, the allegation stated that Mr. Shanahan tried to force the Commandant of the Marine Corps, General Robert Neller, to buy Boeing F/A-18s, and threatened to cut other Air Force programs unless the Air Force Chief of Staff, General David Goldfein, supported buying Boeing F-15Xs," it adds.

However, the OIG found "no evidence" that Shanahan "made any comments pressuring General Neller, General Goldfein, or anyone else, about purchasing F-15 or F/A-18 aircraft, as alleged."

Additionally, the report notes that Sen. Elizabeth Warren's staff forwarded three additional allegations to the OIG on February 14, 2019, asserting Shanahan pressured the Military Services to purchase Boeing aircraft.

Those allegations were also not substantiated by the OIG.

But while Thursday's report seems to indicate that Shanahan has cleared a major hurdle blocking him from becoming the official nominee, there are still lingering questions on Capitol Hill about whether he is the best choice to fill the role vacated by former Defense Secretary James Mattis.

Several Republicans told CNN earlier this month that they're concerned about Shanahan's ability to get through Senate confirmation. There's also a fear over a lack of willing alternatives for one of the most difficult Cabinet jobs. "It's hard to find someone to take it," one senior Republican in Congress has told CNN.

Even before the investigation began last month, delaying Shanahan's anticipated nomination for the permanent position, Republicans on the Hill were concerned about his fitness for the job. "He has not impressed anybody on the Hill," said one Republican congressional aide said of Shanahan. "I've not heard a single person defend this pick."

Trump has been leaning toward nominating him to take the job permanently since January, but the President refrained from doing so before the inspector general's office began its investigation in March.

Two administration officials claimed the initial delay was due to a backlog of other acting positions that needed permanent nominations. But a Republican aide on Capitol Hill says there were also concerns about getting enough support for Shanahan in the Senate.

"I think he's going to have some trouble on the confirmation side," said the aide.