A Japanese bullet train operator is investigating what caused two linked Shinkansen trains to uncouple while moving at high speed. None of the 320 people on board the trains were injured.

A coupler on the Tokyo-bound two-train service on the Tohoku Shinkansen line became unhinged while it was running between Furukawa and Sendai stations in the northeastern prefecture of Miyagi on Thursday morning.

A braking system automatically kicked in, forcing the trains -- Hayabusa and Komachi -- to come to an emergency stop. None of their cars derailed.

The line's operator, East Japan Railway, says this is the first time that its coupled Shinkansen trains have detached while in motion.

The incident prompted the suspension of services between Tokyo and Shin-Aomori stations for about five hours. Roughly 45,000 passengers were affected.

The company, also known as JR East, says its investigation revealed that the trains were traveling at a speed of about 315 kilometers per hour when the uncoupling occurred.

But it adds that the coupler was designed to detach only when the trains slow down to a speed of 5 kilometers per hour or below.

The company says no issues were uncovered during inspections of the trains right before they were put into operation for the day.

It also says the procedure to link the trains went as usual at Morioka Station in Iwate Prefecture. It adds that no external abnormalities were detected on the trains after they came to a halt.

The company quotes the driver as saying no unusual noise was heard or shaking felt shortly before the coupler became unhinged. The driver also said emergency braking was suddenly applied.

The company is trying to identify the cause of the incident. It is also conducting emergency visual inspections of all its Shinkansen trains that are coupled for operations.