Local metro authorities are investigating the cause of a subway glass door that suddenly shattered to pieces at its Laoximen station along the Line 10 subway, after the incident occurred in Shanghai Tuesday for a second time in a week.?

A glass door protecting passengers from incoming trains gave out Tuesday before 6 am, when the subway opens for the day.

No one was injured, however, and the affected area was cordoned off for the safety of passengers, said Lan Tian, a press officer for the city's subway operator, Shanghai Shentong Metro Group, Tuesday.

"Workers will replace the broken glass tonight, after the subway closes," he told the Global Times Tuesday. "We can't do it now because the repairs can only be done in the tunnel."

Lan said that authorities were unsure of the cause of the latest two incidents, saying that the problem is being looked into and that the company supplying the doors has been contacted.

But he said that passengers need not worry about their safety in the meantime as the doors are made from a special material that prevents the glass from shattering into large pieces, reducing risk to those nearby.

Last week, a similar incident occurred at Taopu Xincun station along the Line 11 subway during the morning rush hour, but no injuries were reported from the incident either.