Malaysian Plane -- Cell Tower Could Be the Key In Mysterious Disappearance

A cell tower in Kota Bharu could be the missing puzzle piece in the mysterious disappearance of Malaysia Airlines Flight 370 last weekend.

There's a theory ... the aircraft doubled back over Malaysia after it began to cross the South China Sea, and crashed off the opposite coast instead ... somewhere in the Malacca Strait.

Malaysian officials have rejected the theory -- but if it's true (as some still believe) the plane would have passed over the city of Kota Bharu, which contains a powerful Celcom cellular phone tower.

Credible mobile analysts tell TMZ, if the plane flew over the tower there's a high likelihood the tower would contain data transmitted by cell phones inside the aircraft ... if the phones were on.

Roughly 30% of passengers leave their phones ON during flights -- this according to published reports.

So ... the cellular tower could be the critical piece of evidence to either prove or disprove the U-turn theory.