White House special advisor Jared Kushner, son-in-law of President Donald Trump, boasts an enormous portfolio of domestic and worldwide responsibilities despite having no experience in politics before the 2016 campaign.
The FBI finally cleared Kushner after more than 17 months of vetting, The New York Times reported.
The delay in Kushner's case was caused by a backlog in the new administration and Kushner's extensive financial wealth, which required lengthy review, Lowell said.
In addition, Kushner was interviewed for a second time last month by the office of special counsel Robert Mueller, who is investigating potential ties between the Trump campaign and Russian Federation.
The wealthy NY real estate developer had to refile the national security questionnaire required of all prospective White House employees after making a number of omissions.
He said his client was "looking forward to continuing to do the work the president has asked him to do". The future of Kushner's security clearance was uncertain after chief of staff John Kelly put an end to top-level interim clearances at the White House earlier this year.
Mr Kushner, who serves as Mr Trump's adviser on the Middle East, had his access downgraded in February pending the completion of background checks. But attorney Edward MacMahon, who specializes in national security matters, suggested that the FBI would not have signed off on such a clearance if Kushner was a target of a ongoing criminal investigation.
The questions covered issues related to whether there was collusion between Russian figures and the Trump campaign, and any contacts during the Trump post-election transition period to the presidency, among other issues, the source said. He had previously sat for an interview last November that was largely focused on former national security adviser Michael Flynn, who soon after pleaded guilty to charges of making false statements to the Federal Bureau of Investigation.
The news came hours after US Secretary of State Mike Pompeo declined to comment at a congressional hearing on Kushner's ability to operate as a Mideast peace envoy without access to high-level, classified information.
"In each occasion, he answered all questions asked and did whatever he could to expedite the conclusion of all the investigation", Lowell said.
Some had expected Kushner might not receive a permanent clearance for the duration of Mueller's investigation.
Kushner had to file three updates to his national security questionnaire, a form that guides the Federal Bureau of Investigation background check and asks for information about a person's employment history, finances, family, travel and other matters. He submitted another addendum in mid-May 2017 detailing more than 100 calls or meetings with representatives of more than 20 countries.
Kushner was with Trump in New Jersey the weekend before Comey was sacked, and he was among the attendees at a June 2016 meeting at Trump Tower with a Russian lawyer at which the president's oldest son was told he would negative information about Hillary Clinton.