Engineers working on the closed Delaware River Bridge face the task of determining whether a failed beam was an isolated case or something that could happen elsewhere on the structure.

"They'd be interested in knowing if this is a systemic issue," said Carl DeFebo Jr., spokesman for the Pennsylvania Turnpike Commission.

The crack completely sheared through a truss beneath the westbound deck of the 1.2-mile-long bridge on the Pennsylvania side of the structure and has forced the closure of the bridge, which carries about 42,000 vehicles a day.

Authorities could not estimate when the bridge might reopen, and are reserving judgment on what caused the crack. But engineering experts say pictures of the fissure show signs of holes mistakenly drilled into the beams and then filled with plug welds. That was an approach not uncommon in the 1950s, when the bridge was built, but one modern engineers do not recommend because it can create a weak point in a steel beam. The bridge opened to traffic in May 1956.

The Turnpike Commission would not confirm that was the cause of the break, saying its engineers were withholding judgment on a cause while awaiting the results of tests on metal sampled from the crack, which could take up to two weeks to receive.

The bridge's history is going to be part of engineers' review, though, DeFebo said. Among the information engineers will seek from records are the source of the steel used in the construction, whether there was a flaw in the material or the manufacture, and whether those flaws could exist in other parts of the bridge's supporting structure.

A bridge history the Turnpike Commission posted online showed the bridge underwent major repair work in the 1970s and 1980s, and from 1998 to 2001. A $61 million project that began in 2012 to conduct major reconstruction and repairs is ongoing, according to Turnpike Commission data. The work includes  repairing structural steel, replacing suspenders, and installing cameras and security gates.

Authorities have emphasized the crack was never noticed throughout the work, during a 2014 inspection, or during the recent painting. That, along with the lack of rust and the clean break visible in pictures, indicates it was a sudden event, possibly within the last month.

Chunks of metal were cut away from the affected beam for sampling, and sensors are measuring the load the bridge is carrying. Video monitors are also continuously trained on the bridge.

Officials had initially said an engineer inspecting a new paint job on the bridge spotted the crack, but on Monday, New Jersey Turnpike Authority spokesman Tom Feeney said the first person to notice the fissure was a worker removing scaffolding from the recently completed site of painting. That person then notified staff, and the bridge was closed to traffic by Friday night. The bridge is jointly administered  by the two states' turnpike authorities.

Traffic Monday morning was not as bad as expected, DeFebo said. Some of the longest backups on detours were on the Scudder Falls Bridge on I-95, where traffic backed up for six miles during rush hour. It usually backs up no more than two miles. DeFebo still urged commuters to avoid driving their own cars if possible. He encouraged car pooling, working from home, or shifting schedules to arrive at work later than usual.

A temporary splice — a plate held in place by bolts — was installed by Cornell Crane & Steel in Westville by  Monday, stabilizing the cracked beam, but turnpike authorities are anticipating that a full repair will likely take a significant amount of time.