Kobe Bryant's helicopter pilot was most likely severely disoriented by fog seconds before slamming into a hillside in January -- that's what federal investigators are now saying.

In new documents released Wednesday ... NTSB officials say they have evidence showing Ara Zobayan thought he was ascending when the chopper was actually going downward.

In the docs, investigators claim moments before the crash ... Zobayan told air traffic control he was "climbing to four thousand," and air traffic control asked him what he intended to do once he got to that altitude.

The NTSB says Zobayan never responded -- he had crashed by that point. Interestingly, according to the docs, the helicopter was actually descending rapidly at the time Zobayan said he was climbing.

Investigators add they believe "the pilot could have misperceived both pitch and roll angles" due to the dense fog in the area.

As we previously reported, Zobayan, Bryant and seven others on board hit the the ground at 184 MPH ... instantly killing everybody.

Autopsy results showed Zobayan was NOT high on alcohol or drugs at the time of the crash ... and in the new docs, investigators say Zobayan's girlfriend told them he was in great shape before the accident.

Vanessa Bryant and surviving family members of other passengers have sued the Zobayan and Island Express, which chartered the helicopter.

Various pilots have said Zobayan should not have been flying at such a high rate of speed in blinding fog.

It should be noted, investigators did NOT reveal an official conclusion on the cause of the crash. A final NTSB report with that information is expected to be released later.