STAMFORD -- Repair crews fixed a cracked rail joint bar last month in the area of track on the Bridgeport border where an eastbound Metro-North train derailed last Friday and was struck by a second train, according to federal investigators.

The joint repaired by the railroad in April is not the same one federal investigators have focused on after finding two pieces of rail broken apart at a joint bar at the scene of the crash last Friday evening which injured 76 people, National Transportation Safety Board spokesman Nicholas Worrell said.

Those two sections were taken to Washington, D.C. last week to be tested further by the NTSB which is conducting an investigation into the cause of the accident.

The remaining members of the NTSB's on-site investigative team returned to Washington, D.C. Thursday night ending the first phase of the investigation, according to Worrell.

Worrell said investigators were still determining if the crash broke the joint bar or the broken joint bar directly contributed to the crash.

Joint bars are metal plates fitted lengthwise to the contour of a rail and bolted to it to keep the rail in place.

Worrell said additional information about the location of the repaired joint bar relative to the crash was not available due to the early stage of the investigation.

"We are very early in our investigation and this will be one of our areas of focus," Worrell said.

Earlier this week, the Federal Railroad Administration said that Metro-North conducted one of its required bi-weekly track inspections in the area of the crash on May 15, two days before the accident that did not find any distressed joints or other track infrastructure. In addition, the agency said the railroad had conducted a more in-depth twice yearly inspection scanning the rails for more serious defects in April, finding no problems.

Metro-North and Connecticut Department of Transportation officials declined comment on the investigation Friday.

Allen Zarembski, a rail engineering professor at the University of Delaware College of Engineering who worked for the American Association of Railroads for 27 years said that the underlying flaws that eventually develop into rail breakages at joints can remain barely detectable for years before rapidly progressing over a series of months to cause a derailment or other accident.

"Rails can last 10 to 20 years until the initiation of a defect," Zarembski said. "By the time you come to the propagation phase it goes to failure much more quickly."

The two M-8 rail sets carrying about 700 passengers collided at 6:08p.m. Friday after the eastbound train derailed and a westbound train sideswiped it before both trains stopped. Officials credited the construction of the M-8 cars, which debuted in 2011 and meet more stringent crash-protection standards, for helping to save lives.

Federal investigators also said Friday that so-called positive train control technology, a more advanced collision avoidance system being mandated for installation on Metro-North and other railroads nationally by 2015, would not have aided in stopping the collision.

Initial information obtained from the onboard data recorders indicate that the eastbound train derailed, came to a stop, and was struck about 20 seconds later by the westbound train, the announcement said.

During an interview, the engineer of the eastbound train told investigators that he observed something not right with the track ahead as he approached the Interstate 95 overpass, and that his train had ground to a halt before being struck, according to the announcement.

Metro-North Railroad and the Long Island Rail Road are seeking to jointly contract to install the mandated positive train control system along their tracks, and expect to select a contractor within this year, spokeswoman Marjorie Anders said.

Working through wireless radio frequency spectrum, a positive train control monitors the position and speed of trains throughout a railway and transmits necessary orders to on-board systems when it is determined that a train needs to stop to avoid a crash. If an engineer does not take immediate action to break, the system overrides his control and stops the equipment.

Last month, Metro-North informed the Federal Railroad Administration in a report that various technological challenges including the need to acquire more "radio frequency spectrum," to run positive train control in some service areas would prevent them from adopting the new system by the 2015 deadline.

The railroad needs to acquire more radio frequency spectrum in both Fairfield and New Haven counties to make the system operate effectively, Anders said.

"In other words, it is not technologically possible to comply with the federal mandate to have a functioning, systemwide PTC in place by the end of 2015," Anders said.

Worrell said that NTSB investigators have collected a wide array of evidence including photos, video, data from on-board the railcars, and maintenance and inspection records for the trains involved in the crash.

The investigation is also evaluating the crash for lessons on how to improve safety, including to what degree the sturdiness of the M-8 cars contributed to preventing a loss of life.

On Wednesday, U.S. Sen. Richard Blumenthal announced plans to convene a hearing on rail safety before the Senate Committee on Commerce, Science and Transportation in response to the collision of the two Metro-North trains. No date has been set for a hearing.

"The hearing is going to be very important not only in determining the causes of this collision but in developing support for investments and improvements that are critical for preventing these accidents in the future," Blumenthal said. "I've said it before and I really meant it that this accident can be a teaching moment."