By Andy Pasztor

The Senate Commerce Committee issued strong bipartisan complaints about what lawmakers alleged has been persistent foot-dragging by the Federal Aviation Administration in providing internal documents related to the agency's initial certification of Boeing Co.'s embattled 737 MAX jets.

Sen. Roger Wicker of Mississippi, the Republican chairman of the panel, in a shift from earlier, less-critical public statements, said during a hearing Wednesday he was extremely disappointed and frustrated by senior FAA officials for what he called their "record of delay and nonresponsiveness" turning over materials to the panel.

FAA chief Steve Dickson also clashed with lawmakers for failing to embrace proposed changes seeking to significantly reduce industry's clout in certifying the safety of new aircraft designs.

Sen. Wicker, complaining that only about half of the documents and emails requested nearly a year ago have been provided so far, blasted the FAA for "stonewalling" such oversight requests and trying to "deliberately keep us in the dark."

Sen. Wicker and his staff had suggested, over months leading up to Wednesday's hearing, that they were generally satisfied with the FAA's overall level of cooperation and didn't plan to conduct an extensive investigation, on their own, looking into certification mistakes that led to the grounding of the MAX fleet in March 2019 following two fatal crashes that took 346 lives. Committee staffers had repeatedly stressed that the Senate panel was willing to let the House Transportation Committee lead the way in pursuing detailed congressional oversight of the FAA's actions involving the MAX.

But the tone changed substantially Wednesday, with Sen. Wicker leading the criticism of the FAA and pushing a bipartisan bill that seeks to step up agency oversight of new aircraft design approvals. The bill, among other things, would give the FAA more leeway to appoint, supervise and ultimately control industry experts authorized to act on the FAA's behalf in assessing the safety of onboard systems.

Mr. Dickson told the committee he was committed to cooperating with its continuing inquiry. "It's a responsibility I take very seriously," the FAA chief testified, adding, "We are going to redouble our efforts" to turn over additional documents.

He stopped short, though, of pledging to turn over some of the sensitive internal FAA documents and safety assessments sought by the panel or its members. In certain instances, he said, such disclosures could interfere with ongoing investigations by the agency, the Transportation Department's inspector general or other groups.

Mr. Dickson also clashed with some lawmakers over some of the more far-reaching provisions in the pending bill. Responding to broad questions about proposed changes to the FAA's certification system, Mr. Dickson balked at endorsing specific language that would give FAA managers greater day-to-day authority to monitor safety assessments of new aircraft designs. The Senate bill also creates enhanced whistleblower protections if manufacturers attempt to unduly pressure agency safety reviews.

Sen. Richard Blumenthal, the Connecticut Democrat who has been one of the agency's most outspoken critics, said only a fundamental revamping of the certification process would ensure future passenger safety. Addressing Mr. Dickson, Sen. Blumenthal said the FAA's "culture of secrecy has only been aggravated under your tenure" and later in the hearing slammed prior FAA decisions, saying, "We need an institutional watchdog, not a lap dog."

Sen. Edward Markey, a Massachusetts Democrat who also has been critical of the FAA's handling of the MAX, told Mr. Dickson his lack of transparency was a significant impediment to improving the certification process. "You're part of a coverup," Sen. Markey said.

Texas Republican Sen. Ted Cruz clashed with Mr. Dickson about Boeing's truthfulness with agency officials during certification of the MAX. After the FAA chief conceded Boeing had provided incomplete and fragmented information to the FAA, the Texas lawmaker snapped: "What have you been doing if not finding out whether they lied, and what they lied about?"

Mr. Dickson told the committee that no FAA employees have been fired or disciplined as a result of the mistakes the agency made in authorizing the MAX to fly with what turned out to be a flawed flight-control system that misfired and put the two airliners into fatal nose dives.

Mr. Dickson and his team, prompted by earlier congressional criticism and a series of independent reports pinpointing the FAA's missteps, have jettisoned incorrect engineering assumptions and are moving to correct organizational lapses that led to the MAX crisis.

Until a few days ago, the legislative package drafted by the committee's Republican staffers sought to mandate only changes under way inside the agency. The latest draft goes further, however, envisioning an overhaul of the way the FAA uses industry officials to vet safety issues and likely pilot responses to automated cockpit features. Sen. Maria Cantwell of Washington, a Democrat who represents major Boeing production sites, said the bill would "end any semblance of self-certification" by plane makers

Despite emerging bipartisan consensus in the Senate, prospects for final passage of a bill this year could run into trouble due to House Democrats' championing of more sweeping changes. Rep. Peter DeFazio, the Oregon Democrat who chairs the House Transportation and Infrastructure panel, for months has advocated provisions that would bar Boeing and other plane makers from building multiple versions, or derivatives, of the same basic model potentially spanning decades. Origins of Boeing's 737 family of jets stretch back more than 50 years.

In a statement Tuesday, one of Rep. DeFazio's staffers said the House committee "has made a number of discoveries that reveal a broken system in need of serious change," adding that the panel intends to complete its investigation and majority Democrats will introduce a bill in the near future.

After the hearing, the FAA issued a statement reiterating that returning the MAX to the air will be based strictly on safety considerations rather than arbitrary schedules. The planes aren't expected to fly again until sometime in the fall, at the earliest.

Write to Andy Pasztor at andy.pasztor@wsj.com