Boeing jets that crashed in Indonesia and Ethiopia in recent months lacked two safety features, in part because Boeing charged extra for their installation, the New York Times reported Thursday.

Lion Air and Ethiopian Airlines opted not to install an angle of attack indicator or an angle of attack disagree light onto the jets involved. The angle of attack indicator detects the degree to which the jet’s nose is tilted, and the angle of attack disagree light alerts pilots if the jet’s sensors are sending contradictory signals. While the cause of the accidents involving the two 737 Max 8 jets is not known, investigators are reportedly examining a stall-prevention software system and faulty data sensors. If a system malfunction stemming from faulty sensors caused the Lion Air crash, it’s possible those two optional safety features would have prevented the disaster.

According to the Times, those two features are not expensive, but Boeing makes much of its profit by hiking the prices for these add-ons. Many are for comfort, but some, such as additional fire extinguishers and navigation features, have more to do with safety. Boeing will now include the disagree light in all new 737 Max jets, but the indicator will remain an optional feature.

The Federal Aviation Administration, which is Boeing’s main regulator, did not mandate either safety feature as part of its certification for the Boeing jets. Ethiopian Airlines has said its pilots followed FAA and Boeing guidance. On Wednesday, the Times reported that the pilot for the Lion Air flight had not trained on a 737 Max 8 simulator, possibly because Boeing had said that pilots did not need much training for the new jet. The Times also reported that the black box recording revealed the pilots had struggled to figure out how to handle the malfunction. Little information has yet been released about the Ethiopian Airlines crash.

On Tuesday, the Department of Transportation announced it had launched an audit into the FAA’s certification process. Boeing maintains that it had met all of the FAA’s regulatory requirements. And on Thursday, the FBI joined the criminal investigation by federal prosecutors from the Justice Department into the FAA’s certification of the jets.

Investigators have also asked Boeing for information about training manuals for pilots, as well as other safety procedures and the data it shared with the FAA. They are also examining the ways the company marketed the jets.

On Wednesday, Boeing said it had developed a software patch and pilot training program to address the issues identified from the Lion Air crash. All Boeing 737 Max jets worldwide have been grounded for an indefinite period. Boeing is still building the jets, which are the company’s most important product, according to CNN. Initially, Boeing and the FAA had insisted the jets were safe and that pilots with proper training could overcome any system malfunctions.

The Lion Air crash in October killed 189 people. The Ethiopian Airlines crash on March 10 killed 157.