Boeing improperly influenced a test designed to see how quickly pilots could respond to malfunctions on the Boeing 737 Max, and Federal Aviation Administration officials may have obstructed a review of two deadly crashes involving the plane, Senate investigators say.

In a report released Friday, the Senate Commerce Committee also said the FAA continues to retaliate against whistleblowers. The FAA's parent agency, the Transportation Department, has also hindered investigators by failing to turn over documents, it said.

The report follows a similarly scathing review of the FAA by a House panel earlier this year. Both grew out of concern about the agency's approval of the Boeing Max.

In a statement, the FAA said the report “contains a number of unsubstantiated allegations" and defended its review of the Max, calling it thorough and deliberate.

“We are confident that the safety issues that played a role in the tragic accidents involving Lion Air Flight 610 and Ethiopian Airlines Flight 302 have been addressed through the design changes required and independently approved by the FAA and its partners,” the agency said.

Boeing didn’t comment on specific allegations.

“We take seriously the Committee’s findings and will continue to review the report in full," the Chicago-based company said.

All Max planes were grounded worldwide after crashes in Indonesia and Ethiopia killed 346 people. Following a lengthy review of Boeing changes, the FAA last month approved the plane to fly again if airlines update a key flight-control system and make other changes.

The Senate report, however, criticized a key part of the FAA review. It said that Boeing “inappropriately influenced" FAA testing of pilot-reaction time to a nose-down pitch of the plane.

According to a whistleblower who was an FAA safety inspector, Boeing representatives watched and gave advice to help test pilots in a flight simulator respond to the problem in a few seconds. The reaction of three flight crews was still slower than Boeing had assumed, according to the report. Each time the plane would have been thrown into a nose-down pitch, although recovery would have been possible, the investigators said.

In the two Max crashes, a failure of the key flight system, called MCAS, pushed the nose down repeatedly, sending the planes into fatal dives.

The FAA countered that it was an FAA pilot who discovered a separate computer issue in the plane, a flaw that took Boeing additional months to fix.

Investigators also said an FAA division manager was first invited, then excluded from a review of the Max crashes even though his position normally would call for him to participate in the review. The official said he believes he was excluded to shield FAA from criticism.

The committee chairman, Roger Wicker, R-Miss., called the investigators' findings troubling.

“The report details a number of significant examples of lapses in aviation safety oversight and failed leadership in the FAA,” Wicker said in a statement. "It is clear that the agency requires consistent oversight to ensure their work to protect the flying public is executed fully and correctly.”

Wicker and the panel's top Democrat, Maria Cantwell of Washington, have introduced legislation to make changes in FAA's process for certifying new planes. The House passed a similar but more far-reaching bill offered by Transportation Committee Chairman Peter DeFazio, D-Ore.