Indonesian investigators have found that design and oversight lapses played a key role in the October 2018 crash of Boeing Co's 737 MAX jet that killed all 189 people aboard, the Wall Street Journal said on Sunday.

The draft conclusions, expected to be the first formal government finding of flaws in the design and U.S. regulatory approval, also identify a string of pilot errors and maintenance mistakes as causal factors of the Lion Air crash, the WSJ here said.

The Boeing 737 MAX has been grounded since March in the aftermath of two fatal crashes in five months.

A Boeing spokesman did not comment on the newspaper report but said the plane maker continued to offer support to the investigating authorities as they complete their report.

Soerjanto Tjahjono, the head of Indonesia's transport safety committee, told Reuters he could not comment before the release of the final report, which is expected by early November.

He said several stakeholders, but not all, had already provided feedback on a draft of the final report that has not been released publicly.

The draft was circulated to parties including Boeing, Lion Air and the U.S. Federal Aviation Administration on Aug. 24.

"There are stakeholders that have sent their answers to us and we are evaluating them," Tjahjono said.

U.S. air crash investigators are readying to announce a handful of separate safety recommendations, from bolstering pilots' manual flying skills to boosting FAA vetting of new aircraft designs, the WSJ added.

Around month-end, the U.S. National Transportation Safety Board (NTSB) is expected to call for improvements to cockpit training and crew decision-making and focus on potential changes to certification of new airliners, the newspaper said.

The NTSB declined to comment on the WSJ report but said it planned to release recommendations on the FAA's certification program sometime in September.

The FAA welcomed the scrutiny from safety experts and looked forward to their findings, it said in a statement.

"We continue to work with other international aviation safety regulators and will carefully consider all recommendations," it added. "The FAA will incorporate any changes that would improve our certification activities."