The New York commuter train that derailed Sunday morning, killing four people, was going 52 miles over the speed limit at the time of the derailment, according to National Transportation Safety Board (NTSB) investigators.

More than a dozen of the 63 people injured in the incident remain hospitalized, some with critical injuries.

An examination of one of two black box data recorders recovered from the wreckage at the Bronx crash site found that the seven-car Metro-North train was speeding both in the 30 mph zone at the sharp curve where it derailed as well as in the 70 mph speed limit zone preceding it. The train’s engineer, Bill Rockefeller, is a 20-year-veteran of the company.

The NTSB’s Earl Weener said the train's brakes were applied five seconds before the derailment but declined to speculate on whether the accident was caused by human error or mechanical failure.

The train did not have the “positive train control” required by a federal regulation on all trains by the end of 2015, although the Metropolitan Transportation Authority which controls the Metro-North railroad said it is in the process of installing it. The technology uses computer and satellite tracking to overcome human error.

The regulation – which came on the heels of two deadly train crashes in 2008 -- been met with opposition by the railroad industry, which estimates that installing it will cost billions.  A Government Accountability Office report issued several months ago found that most railroads will not meet the deadline.

The NTSB expects to be in New York for the first phase of its investigation for at least another week. The Federal Railroad Administration is also investigating the accident.