The China Eastern Airlines plane crash that killed 132 people is believed to have been caused by an intentional act, according to U.S. officials who spoke to ABC News.

The Boeing 737-800 passenger jet was flying from Kunming to Guangzhou on March 21 when it plunged into a mountainous area in Guangxi, China. All 123 passengers and nine crew members were killed.

The officials who spoke to ABC News point to the plane's flaps not being engaged and landing gear not put down. The near-vertical descent of the plane, they believe, would've required intentional force.

The plane slammed into the ground with such force that it created a 66-foot deep hole in the ground, according to Chinese officials.

Investigators also looked into one of the pilots' personal lives and background and believe he may have been struggling through certain issues right before the crash, ABC News has learned.

The U.S. National Transportation Safety Board said all information on the investigation will come from their counterparts in the Civil Aviation Administration of China, but regulators and Boeing have not flagged any mechanical issues. Sources said Chinese investigators also haven't flagged any mechanical issues.

"The NTSB will not be issuing any further updates on the CAAC's investigation of the China Eastern 5735 crash," the NTSB said in a statement. "When and whether CAAC issues updates is entirely up to them. And I haven't heard anything about any plans for them to do so."

The first black box, the cockpit voice recorder, was found on March 23, while the flight data recorder was found on March 27.

Early data showed the airliner plunged from 29,000 feet to 8,000 feet, leveled off and then went into a freefall. One video showed the plane nose-diving into the ground.

ABC News