The Pentagon’s watchdog cleared acting Secretary of Defense Patrick Shanahan in its investigation into whether he violated any ethics rules by promoting his longtime former employer Boeing Co. while serving in the Trump Administration. The reprieve removes a major hurdle for President Donald Trump to nominate him as Defense Secretary, a post that has been empty for 112 days since James Mattis resigned last December.

The Department of Defense Office of Inspector General had launched the probe in the face of allegations he pushed military leadership to purchase more Boeing-made F-15X fighter jets and openly criticized Boeing-rival Lockheed Martin Corp. during government meetings. The allegations were based on referrals from an anonymous Senate Armed Services Committee attorney, Massachusetts Sen. Elizabeth Warren’s staff, several media reports and an outside watchdog group, called Citizens for Responsibility and Ethics in Washington.

The inspector general was able clear Shanahan in a just a month, which is lightning-quick for an Inspector General’s investigation that often run months-long. Over that time, investigators said they interviewed Shanahan and 33 witnesses, including top Pentagon brass, while also poring over more than 7,300 pages of classified and unclassified documents.

“We found that the allegations were not substantiated and that Mr. Shanahan fully complied with his ethics agreements and ethical obligations regarding Boeing and its competitors,” the inspector general said in its 47-page report, which was made public Thursday.

Shanahan, who came to the Pentagon after spending more than three decades at Boeing, has routinely fended off questions about potential conflicts of interest with the aerospace company, which also happens to be one of the largest suppliers for the U.S. military. In March, he told Congress that he welcomed any such investigation into his actions in office.

The investigation came after Citizens for Responsibility and Ethics in Washington wrote a nine-page complaint in March to the Pentagon’s inspector general urging the agency to scrutinize the relationship. In its complaint, the group cited a Politico report from January that said Shanahan had been promoting Boeing while criticizing the F-35 Joint Strike Fighter, a Lockheed Martin program. The plane was “f-cked up,” he reportedly said, and Lockheed Martin “doesn’t know how to run a program.”

However, the inspector general said in its report that Shanahan told investigators that he never said the F-35 fighter jet, itself, was “f-cked up.” He told them the aircraft is “awesome.” However, Shanahan told investigators that he said the F-35 program was “f-cked up.” Evidently, that point of distinction was important, because the inspector general found his comments were “substantive, related to the program’s performance, and were consistent with comments about the F-35 program made by other senior Government officials.”

The inspector general’s office interviewed those most likely to have encountered Shanahan’s alleged bias, including Shanahan’s old boss, Mattis; Gen. David Goldfein, the chief of staff of the U.S. Air Force; Marine Commandant Gen. Robert Neller. Investigators also reviewed Shanahan’s emails and other correspondence for terms related to Boeing and Boeing products.

“We determined that Mr. Shanahan did not make the alleged comments and did not promote Boeing, or disparage its competitors,” the inspector general’s report said. “While Mr. Shanahan did routinely refer to his prior industry experience in meetings, witnesses interpreted it, and told us, that he was doing it to describe his experience and to improve Government management of DoD programs, rather than to promote Boeing or its products.”

The inspector general also cleared Shanahan of any wrongdoing in three other issues related to Boeing, which were also proven erroneous.

“I have never heard him mention, nor have I had a conversation [with him] about Boeing, and to the extent that the issue has come up we did Congressional testimony together,” Chairman of the Joint Chiefs of Staff, General Joseph Dunford told investigators.

Lieutenant Colonel Joe Buccino, a spokesman for Shanahan, did not comment on the findings. “Acting Secretary Shanahan’s focus remains on our global operations, our strategy, and our service members, civilians, and families,” he said in a statement.

The resolution of the allegations takes on increased importance because Shanahan, 56, is the leading contender to become Defense Secretary. He has the dubious distinction of being the longest serving “Acting Defense Secretary” in the nation’s history. The path is now clear for the nomination, now that this dark cloud has passed.

A Washington state native, Shanahan initially stepped into the No. 2 spot at the Pentagon in 2017 after leaving his business career at Boeing. A mechanical engineer by trade, with two advanced degrees from the Massachusetts Institute of Technology, Shanahan quietly turned around some of Boeing’s most troubled programs, including a multibillion-dollar missile defense system and the problem-plagued 787 commercial airliner. His success earned him the nickname, “Mr. Fix-It,” inside the company.

At the Pentagon, Shanahan has assumed the role of a technocrat, leading efforts to cut wasteful spending and create a standalone Space Force – a priority for Trump. He’s maintained a low public profile thus far, and he has not forged strong ties around Washington like Mattis, a retired four-star Marine general, had garnered.

Shanahan, who has relatively little foreign policy experience, would assume the top job at a time of historic change around the world. He’ll have to juggle the continued fight against terror groups in the Middle East, Russia’s renewed resurgence in Europe and China’s muscular rise in Asia. Add to that the litany of other issues putting pressure on the liberal international order that won the Cold War and advanced American interests for a half century.

Shanahan now has a daunting task of navigating this minefield, while also managing the expectations of a demanding Commander-in-Chief. For instance, the Pentagon entanglements in the Administration’s immigration policy have worried both parties on Capitol Hill, and top U.S. military leaders have faced tough questions from Congress about the months-long troop presence along the southern border with Mexico. The Pentagon is also on the hook to help shelter 5,000 migrant children caught crossing the U.S. southern border without a parent.

“It shouldn’t come as a surprise that we’ll provide more support to the border,” Shanahan told reporters on April 12. “And the way I tend to frame that is, it’s — our support is very elastic.”

Write to W.J. Hennigan at william.hennigan@time.com.