Washington (CNN) US officials say the recent government shutdown played a part in the delay of Boeing's software update for its 737 Max aircraft, which has been grounded by airlines and countries around the world after two deadly accidents in the past five months, the Wall Street Journal reported.

Boeing announced Monday it had been working on a software fix for the 737 Max aircraft over "the past several months and in the aftermath" of Lion Air Flight 610 that went down in late October over the Java Sea off Indonesia, killing 189 passengers. The process was underway before the Ethiopian Airlines Flight 302 crash over the weekend, which killed 157 people and also involved a 737 Max. Both flights crashed minutes into their journeys.

The circumstances of the crashes remain under investigation.

Boeing, which said it was working with the Federal Aviation Administration "on development, planning and certification of the software enhancement," said the fix will be on 737 Max planes no later than April.

But the company's software update had initially been expected in early January, the Journal reported.

Citing people familiar with the details, the Journal reported that discussions between the FAA and Boeing about the software fix dragged on, in part because of "differences of opinion about technical and engineering issues."

Boeing and FAA officials also couldn't agree on how extensive the software enhancement should be, the newspaper reported. US officials also said the recent government shutdown, the longest in US history, delayed work on the update for five weeks, the Journal noted.

Later on Wednesday, acting FAA Administrator Daniel Elwell denied that the shutdown played a role.

"We just got confirmation that the shutdown did not cause any delay in work on the software -- the software addition to the MAX," Elwell said.

Sen. Richard Blumenthal, who sits on the Senate Commerce, Science and Transportation committee, said the shutdown "absolutely aggravated and exacerbated the failures of the FAA and insisted on a quicker time table for installing new software dealing with the sensors."

"This problem is not a technological equivalent of rocket science," the Connecticut Democrat told reporters on Capitol Hill Wednesday. "They had the new software. They knew of the problem with the sensors. The airline should be held accountable, but the FAA has a responsibility to act right away."

The FAA was one of the agencies affected by the government shutdown, which began on December 22 and spanned 35 days. The agency had to recall thousands of its aviation inspectors from furlough over safety concerns.

The FAA determined that the delay in the update was acceptable because its experts and Boeing deemed there was no imminent safety threat, an unnamed source briefed on the discussions told the Journal.

Boeing declined to comment to CNN about the Wall Street Journal's reporting.

President Donald Trump announced on Wednesday that the US would be grounding 737 Max 8 and 9 planes effective immediately.

The FAA and Boeing say the software upgrade that's due in April will give pilots greater control in case problems emerge with the planes' safety systems.

Boeing said the company plans to include an update to the Maneuvering Characteristics Augmentation System (MCAS) flight control law.

"The enhanced flight control law incorporates angle of attack (AOA) inputs, limits stabilizer trim commands in response to an erroneous angle of attack reading, and provides a limit to the stabilizer command in order to retain elevator authority," Boeing said.

The software fix will also include updates to pilot displays, operation manuals and crew training.

A new flight control system

The maneuvering characteristics augmentation system is a new feature to Boeing's Max planes. It's a system that automatically lowers the nose of the plane when it detects from its external angle of attack sensors that the aircraft is flying too slowly or steeply, and at risk of stalling.

AOA sensors send information to the plane's computers about the angle of the plane's nose relative to the oncoming air to help determine whether the plane is about to stall.

The pilots on the Lion Air flight had repeatedly fought to override the MCAS, which pulled the plane's nose down more than two dozen times, according to a preliminary report released in November by Indonesia's National Transportation Safety Committee.

The report said the system was responding to incorrect data transmitted by an AOA sensor, which had proved faulty on earlier flights and had been replaced.

Lion Air's operational director claimed Boeing's operational manual for the Max 8 didn't contain adequate information about the new MCAS system. Boeing Chief Executive Dennis Muilenburg told Fox Business Network at the time that the information was available as part of the training manual.

A week after the Lion Air crash, Boeing warned airlines about how to address erroneous readings related to the plane's external sensors.

This story has been updated.