Boeing was able to oversee much of the certification of its software that has been linked to two fatal 737 Max crashes, the acting head of the Federal Aviation Administration told Congress on Wednesday.

He said that the FAA initially oversaw the software's certification but that it gave more authority to Boeing "when we had the comfort level" that it could oversee the system.

The FAA delegates parts of the plane-certification process to aircraft manufacturers as part of a longstanding policy that is now under increased scrutiny.

The FAA defended the policy as one that produces safe aircraft, saying that undertaking the whole certification process itself would require 10,000 more employees and $1.8 billion more in funding.

But lawmakers criticized the policy and called for change, with one senator describing it as a system "fatally riddled with flaws" and another accusing the FAA of doing "safety on the cheap."

The Federal Aviation Administration said it allowed Boeing to partially oversee the certification of its 737 Max plane software, the focus of investigations into two crashes that killed almost 350 people.

The FAA delegates parts of the plane-certification process to aircraft manufacturers as part of a policy that has come under scrutiny since the fatal crashes.

Daniel Elwell, the acting administrator of the FAA, told a subcommittee of the Senate Committee on Commerce, Science, and Transportation on Wednesday that the agency initially oversaw the software's certification but gave more authority to Boeing "when we had the comfort level" that it could oversee the system, according to The New York Times.

Read more: Boeing just unveiled how it's going to fix the 737 Max that was grounded after 2 fatal crashes in recent months

Elwell described the process as "part of the fabric of what we've used to become as safe as we are today."

Families of passengers on the Lion Air flight looking at the belongings of passengers at a port in Jakarta, Indonesia. REUTERS/Beawiharta

He said the FAA would need 10,000 more employees and an additional $1.8 billion if it were to be solely responsible for aircraft-safety certifications.

The Department of Transportation's inspector general, Calvin Scovel, announced on Wednesday that it would audit the FAA's certification of the 737 Max, as requested by Transportation Secretary Elaine Chao.

Read more: Europe and Canada are investigating the Boeing 737 Max themselves rather than trusting the US — another apparent snub of American regulators

Chao has described the practice of allowing manufacturers to help certify their own planes as "necessary" and said the FAA sets safety standards for manufacturers.

The close ties between aircraft manufacturers and the FAA have come under increased scrutiny since the Lion Air crash in October and the Ethiopian Airlines crash earlier this month. A focus of the ongoing investigations into the causes of those crashes is an automated control system designed to point the 737 Max's nose down if it detects the plane is flying at too steep an incline.

Elwell, left, and the Department of Transportation's inspector general, Calvin Scovel, arriving at the Senate subcommittee hearing. AP Photo/Andrew Harnik

The policy of aircraft manufacturers helping with their products' certification was first ordered by Congress in 2003 as part of efforts to speed up the certification process and reduce costs. The FAA delegated authority to Boeing in 2009 and now allows more than 80 aviation companies to help certify their products' safety, according to The Washington Post.

Read more: 'We are all humbled': Boeing CEO publishes contrite open letter, as 737 Max crash investigations zero in on software linked to nosedives that brought down 2 planes

Several lawmakers criticized the process on Wednesday, with Democratic Sen. Richard Blumenthal saying he would introduce legislation to reform the system and describing it as "fatally riddled with flaws," according to The Times.

"The fact is that the FAA decided to do safety on the cheap, which is neither safe nor cheap," he said.

Republican Sen. Ted Cruz said that the "close relationship between industry and regulators" threatened to erode the public's trust in the aviation industry, The Associated Press reported.

Democratic Sen. Tom Udall also said that the relationship needed to be questioned and that changes were necessary to ensure that safety "remains the paramount interest, not the quarterly profits of this company."

Sen. Richard Blumenthal. AP Photo/Andrew Harnik

Scovel said the Department of Transportation planned to change the existing system by July, the AP reported.

Mike Sinnett, Boeing's vice president of airplane development, reemphasized on Wednesday that the company was confident in the safety of the 737 Max.

"We are working with customers and regulators around the world to restore faith in our industry and also to reaffirm our commitment to safety and to earning the trust of the flying public," he said, according to the AP.