(CNN) There are signs of a disconnect between the US Federal Aviation Administration and overseas aviation regulators six months after the second fatal Boeing 737 Max crash, raising the prospect that the aircraft may be recertified in some countries but delayed in others -- a situation the aviation industry had hoped to avoid.

A staggered international return to service for the commercial jet would cause headaches for airlines that would need to consider where the aircraft can and cannot fly when planning flight schedules.

Worldwide, regulators are breaking with the tradition of trusting the FAA's expertise and launching independent reviews of the plane. The multiple reviews -- plus a lengthy list of US-based probes, Boeing's delay in finalizing the software fix and pressure from victims' families to conduct a more comprehensive review -- leave it unclear when the troubled jet will fly again.

Those factors could result in a "staggered return to service" for the Max, rather than global regulators acting in concert to approve the plane, a Boeing source acknowledged to CNN. The source noted that "international safety regulators have their own thorough process" at this point for determining whether the plane is safe to fly.

That assessment comes as the head of the European Union Aviation Safety Agency recently laid bare what watchers of the industry have mused for months. He told the European Parliament that the FAA's certification of the Max -- and response to the aircraft's October 2018 crash, when the agency issued instructions to pilots -- has shaken trust in the FAA's judgment among his colleagues worldwide.

"In terms of overall international community and reputation of the FAA, I think the FAA is in a very difficult situation where when they say this is good to go," said Patrick Ky of the European Union Aviation Safety Agency.

"It's very likely that international authorities will want a second opinion -- or a third opinion" before returning the Max to the skies, he said. "And it was not the case like this one year ago. I think that's going to be a very strong change in the overall worldwide hierarchy or relationship between the different authorities."

The FAA told CNN in a statement that it "has a transparent and collaborative relationship with other civil aviation authorities as we continue our review of changes to software on the Boeing 737 Max. Our first priority is safety, and we have set no timeframe for when the work will be completed."

Boeing said in a statement to CNN it is working "diligently with the FAA and global regulators to support the safe return to service of the Max."

"We continue to respond and provide information for requests from the FAA and global regulators and complete the work on the additional software requirement," the statement said. "While the timing for return to service is driven by the FAA and global regulators, there is no change to our best estimate for the start of the return to service in early fourth quarter."

The head of the International Air Transport Association, a trade association of the world's airlines, said the possibility of a staggered return to service could be bad for business.

"We are a bit worried because we do not see the traditional unity among the regulators," CEO Alexandre de Juniac said, expressing concern about how safety regulators acting independently could affecct the industry.

It's been nearly a year since the first 737 Max malfunctioned soon after takeoff from Jakarta, Indonesia, killing all 189 people aboard Lion Air Flight 610. By the time the second Max crashed in March 2019 -- six months ago Tuesday -- the extent and dangers of the plane's automated stability system were coming to light, catching regulators and the aviation world off guard. The second crash killed 157 people.

Boeing had not initially told pilots that the Maneuvering Characteristics Augmentation System, or MCAS, was a new addition to the 737 Max, the latest variation of the decades-old workhorse jet.

In the wake of the Ethiopian Airlines crash, the FAA found itself among the last regulators to order the 737 Max grounded. The agency's acting head said he had been awaiting enough information to make a decision.

Traditionally, the FAA has taken the lead worldwide in regulating Boeing, which is based in the US, and international regulators who trusted the agency's judgment and expertise usually followed its lead.

"Whether they want to state it publicly or not, it's clear many of the world's FAA counterparts no longer feel comfortable simply falling in line behind the FAA's safety decisions," said Peter Goelz, a former managing director of the National Transportation Safety Board who's now a CNN aviation analyst.

A long-standing and unique practice in aviation regulation of deferring some aspects of safety oversight to the manufacturer appeared to leave the FAA in the dark as to what the MCAS system was capable of, a practice that has shaken the confidence of foreign regulators and the general public.

Ky, the European regulator, said his agency has assigned 20 people to review all "safety-critical systems" where it had previously trusted the FAA's analysis. That decision was "not very popular with our American colleagues," Ky said.

Their work has uncovered "significant technical issues," he said, including issues with a system related to MCAS that have not yet been resolved, according to his presentation last week.

The tarnish of the Max accidents have hit Boeing's reputation, too.

"Clearly, Boeing's credibility has been severely damaged. They continually tried to minimize the extent of the problem and advertise the fix was a minor software fix," Goelz said. "This has also severely undercut the FAA's leadership in air safety worldwide."

The family members of 737 Max victims have become vocal proponents for changes at the FAA, and announced plans to hold a vigil outside the US Transportation Department's Washington headquarters on Tuesday. Organizers said they would hold photos of their family members killed in the crash.

While Boeing works on revamping its software -- including for another flaw discovered in early summer during testing -- and its reputation, multiple investigations, including a criminal probe, are ongoing. An international panel convened by the FAA to look at the certification process is expected to file its report soon and recommend changes.

US carriers that fly the Max are hoping regulators give the plane the green light as soon as December, but that date has been in flux. Airlines have gone through multiple rounds of rearranging their schedules, one month at a time.

After Boeing's fix is finalized and submitted in the coming weeks, regulators will review it and schedule test flights, expected later this fall. Then they'll determine the type and amount of additional training that 737 Max pilots will need to complete.

At that point, airlines will need to pull the mothballed planes out of storage, and each will receive a thorough nose-to-tail examination to make sure all systems are working and there are no mechanical or electrical issues. Boeing recently announced that it will hire mechanics to work on that process It will also need to convince skittish customers the planes are safe.

If the 737 Max is not flying in time to meet the volume demands of holiday travel, Goelz predicts that flyers will feel the impact.

"Seats will be harder to get on flights and tickets will be more expensive if these planes aren't available for the holiday rush," he said.