The Federal Aviation Administration said findings from the crash site near Addis Ababa and "newly refined satellite data" warranted "further investigation of the possibility of a shared cause for the two incidents."
FAA acting chief Daniel Elwell said the agency has been "working tirelessly" to find the cause of the accident but faced delays because the black box flight data recorders had been damaged.
The new information shows "the track of that airplane was close enough to the track of the Lion Air flight... to warrant the grounding of the airplanes so we could get more information from the black boxes and determine if there's a link between the two, and if there is, find a fix to that link," Elwell said on CNBC.
In Miami, a large hub for US air travel, passengers formed growing lines to reschedule after canceled flights.
Boeing chief Dennis Muilenburg said he supported the US decision "out of an abundance of caution" but continued to have "full confidence" in the safety of the plane.
The company continues its efforts "to understand the cause of the accidents in partnership with the investigators, deploy safety enhancements and help ensure this does not happen again," Muilenburg said in a statement.
The accounts of the recent crashes were echoed in concerns registered by US pilots on how the Max 8 behaves.
At least four American pilots made reports following the Lion Air crash, all complaining the aircraft suddenly pitched downward shortly after takeoff, according to documents reviewed by AFP on the Aviation Safety Reporting System, a voluntary incident database maintained by Nasa.
It was unclear if US transportation authorities review the database or investigate the incidents. However, the FAA said this week it had mandated that Boeing update its flight software and training on the aircraft.
According to the flight data recorder, the pilots of Lion Air Flight 610 struggled to control the aircraft as the MCAS repeatedly pushed the plane's nose down following takeoff.
The Ethiopian Airlines pilots reported similar difficulties before their aircraft plunged into the ground as they tried to return to the airport.
Andrew Hunter, a defense industry expert at the Center for Strategic and International Studies in Washington, said that, while Boeing and the FAA had good track records on addressing safety concerns, sometimes the combination of automated systems and humans did not work smoothly.
"It is hard to get a system to work seamlessly with human beings," he told AFP.
"The fact the system was fighting the pilot was not an unintended consequence," because it should counteract a pilot error and correcting this is "challenging."
In Ethiopia, distraught families wept and lit candles as they visited the deep black crater where the plane smashed into a field, killing 157 passengers and crew, an AFP correspondent said.
The Ethiopian Airlines 737 MAX 8 was less than four months old when it went down six minutes into a flight from Addis Ababa to Nairobi on Sunday, disintegrating on impact.
Families of the victims from Kenya, China, the United States and Canada, as well as diplomatic staff from embassies, were visiting the crash site.
A dozen airlines have grounded the plane, while Nigeria, Lebanon, Egypt, Serbia, Vietnam, New Zealand and Hong Kong on Wednesday also joined the list of countries to ban it from their airspace.
The European Union and major hubs such as the UAE and Australia had already done so.
American Airlines said it had 24 aircraft affected by the US ban, while Southwest Airlines said it was still confirming the move.
The Max series is Boeing's fastest-selling model.
There are 74 of the planes registered in the United States, and 387 in use worldwide with 59 carriers, according to the FAA.
Low-cost airline Norwegian Air Shuttle has said it would demand financial compensation from Boeing as the implications of the mass grounding for the airline industry remained unclear.
Shares in the company rose Wednesday on Wall Street despite the US order but were still down 10.6 percent since before Sunday's crash.
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