Fifteen months after the grounding of the Boeing 737 Max, lawmakers accused the Federal Aviation Administration (FAA) of stonewalling their investigation into the certification of the troubled jet and introduced bipartisan legislation that if passed will reform the way the agency certifies aircraft.

Wednesday marked the third hearing the Senate Commerce Committee has held on the Boeing 737 Max since it launched its investigation into the Boeing plane following two fatal crashes that killed a total of 346 people.

Ethiopian Airlines Flight 302 crashed near Addis Ababa airport just six minutes after takeoff, killing all on board, in March 2019. It was the second crash involving a Boeing 737 Max within five months. In October 2018, Lion Air Flight 610 was airborne for only 13 minutes before it plunged into the Java Sea off the coast of Indonesia.

Sen. Roger Wicker, R-Miss., in his opening statement expressed “profound frustration” with the FAA’s “lack of responsiveness” to most of the committee’s requests for documents — some of them directly relating to the certification of the Boeing 737 Max.

Wicker said the lack of responses forced committee staff to seek interviews with FAA staff in October, but the “response and handling” of those requests has been “very slow” — resulting in the interviews of four FAA staff members within seven months.

“It’s hard not to conclude your team at the FAA has deliberately attempted to keep us in the dark,” Wicker said, adding he can only assume that the agency’s “stonewalling” of the committee’s investigation “suggests discomfort for what might ultimately be revealed.”

Sen. Richard Blumenthal, D-Mass., told FAA Administrator Steve Dickson he believes since Dickson took control of the agency on Aug. 12 the “culture of secrecy at the FAA has only been aggravated” and that he sees no way he can continue in his job if he fails to be more responsive.

“The fact of the matter is that the FAA has been complicit in these crashes by failing to do more diligent oversight,” Blumenthal said.

Dickson pushed back against the portrayal of the agency as “unresponsive,” telling lawmakers he is “totally committed to the oversight process.”

“I hear your frustration and it is not OK with me,” Dickson said. “I am trying to promote a culture, both within the agency, and really with all of our stakeholders of transparency and openness.”

Boeing’s 737 Max is still the subject of numerous ongoing investigations, including probes by the U.S. Department of Justice and the Securities and Exchange Commission.

Will the 737 Max crashes lead to changes in the way aircraft are certified?

On Tuesday, Sen. Wicker alongside Sen. Maria Cantwell, D-Wash., introduced The Aircraft Safety and Certification Reform Act of 2020.

Under the current Organization Designation Authorization (ODA) program mandated by Congress, some of the aircraft certification process is delegated to manufacturers like Boeing. Critics of the ODA program say the fact that Boeing was able to influence the certification of the 737 Max represents a conflict of interest. However, some defend the program and say that there is sufficient FAA oversight from start to finish during the aircraft certification process.

Dickson referred to ODA as a process that is “based on trust.”

“I would say it’s a trust but verify system,” Dickson explained, “and it’s a privilege, it has to be earned.”

Cantwell said that if passed, the new bill would “revamp the ODA” and “make sure that the FAA stay’s in the driver’s seat of certification.”

“Our bill will end any semblance of self certification,” Cantwell claimed.

When will the 737 Max fly again?

Boeing is aiming to conduct its 737 Max certification test flight(s) with FAA pilots — a key step in the eventual ungrounding of the plane — at the end of the month, sources confirmed to ABC News.

Last week Boeing sent airlines draft, unapproved pilot training materials “to assist” them” in the development of their own training programs,” according to sources at two U.S. airlines.

These are signs that Boeing is getting closer to the end of its already more than year-long revision process. However, even if the FAA certification flight occurs at the end of this month, it is unlikely the MAX will be ungrounded before September, sources familiar with the matter explained.

Dickson repeated in his opening statement on Wednesday that the FAA’s “process is not guided by a calendar or schedule” and that “safety is the driving consideration.”

A former Air Force and commercial pilot, Dickson pledged again in front of lawmakers that he will not sign off on the aircraft until he flies the plane himself and once he is satisfied that he would put his own family on it “without a second thought.”

After the certification flight, the next hurdle is establishing pilot training requirements. Each U.S. airline must also get its own individual training plans approved by the FAA, and train its pilots, which is expected to require some simulator time.

The CEO of the largest U.S. operator of the Max, Southwest CEO Gary Kelly, has said he expects the Max will return to service by the end of the fourth quarter.

ABC News’ Amanda Maile contributed to this report.