Engineers in Pennsylvania and New Jersey faced a mystery as they investigated the cause of a fractured beam on the Delaware River Turnpike Bridge last month.

Poring through more than 50 boxes of records on the bridge's history, in addition to giving the bridge a close inspection, officials have gathered clues but aren't ready to announce a cause.

The answer could have ramifications across the country for the maintenance of America's aging infrastructure. Plug welds, used to patch excess holes, are a leading contender for creating a weak point in the beam, officials said.

"Does it hold some water? Yeah, it holds some water from the fact that the fracture occurred at the plug welds," said Brad Heigel, chief engineer for the Turnpike Commission. "We think there are other pieces of the puzzle that we're still investigating."

If the weld issue proves to be a critical factor in the break of the 60-year-old steel truss, officials responsible for similar bridges across the country may be put on alert, said state officials. In the past, the Federal Highway Administration has informed other states that a flaw found in a bridge could exist in other structures. The FHWA took that action a decade ago after a Minneapolis bridge collapsed, killing 13 and injuring 145.

"Immediately after the  collapse, we sent around a technical advisory to all the state departments of transportation to look at this particular issue," Nancy Singer, a spokeswoman for the FHWA, said of the Minneapolis collapse.

Similar federal guidance could result from the Delaware River Bridge investigation.

"It depends on what we find," she said.

An excess load on the bridge, which carries about 42,000 vehicles a day, also may have played a role, officials believe. A possible explanation they raised: In December 2015, a cashless tolling point on the Pennsylvania side of the bridge's westbound lanes replaced a traditional toll booth, which could have allowed excessively heavy truck traffic to cross it.

It's also possible that an answer may never be found, state officials cautioned.

When the 1.2-mile bridge will reopen depends on the results of ongoing testing of the bridge's metal, information gathered from the investigation, and the success of a challenging effort to hoist the bridge back into position.

Thousands of tons of concrete and steel shifted when the truss beam on the westbound side of the Delaware River Turnpike Bridge split in two. The broken beam is now an inch out of alignment horizontally, and 3/4 of an inch misaligned vertically, turnpike officials said.

Engineers are installing eight towers beneath the damaged span to each hold jacks capable of lifting 600 tons. It will take three days of slow lifting to get the broken beam back where it should be. That's a best-case scenario.

"At the end of the day, when we start jacking this truss, we don't know how it's going to behave," said Heigel.

If all goes well, a permanent plate will be bolted to the split beam to hold it in place. The surrounding beams, some of which have bowed from supporting additional weight, may also need repairs.

More than a dozen contractors have been enlisted so far. Tuesday, the New Jersey Turnpike Authority approved two contracts for a combined $18 million as part of the repair work. The two agencies share responsibility for the bridge.

Repairs are progressing alongside the search into whether the plug welds could pose a larger problem for the bridge, and bridges elsewhere.

Through decades of Delaware River Bridge inspections, including about 15 in the last 30 years, no one was looking for these plug welds. An Inquirer review of reports from biannual inspections conducted in 2010, 2012, and 2014, found references to sheared bolts, corrosion and rust, and gaps between plates, but no mention of the welding. And there haven't been any such references going back to the first inspection conducted in 1968, said Heigel. The current review of documents  includes material dating back to the bridge's construction.

"To this day, we didn't know the plug welds existed," Heigel said.

Bridge officials have sought records from the American Bridge Co., the Pittsburgh-based firm that built the bridge in 1956. The company did not elaborate on its involvement in the investigation, confirming only that it built the bridge.

American Bridge has a long history of bridge construction and repair in Pennsylvania, including participating in the original construction of the Ben Franklin Bridge in 1926, and doing rehab work on that span in the 1980s. The company also worked on the Walt Whitman Bridge, and built Pittsburgh's Three Sisters bridges, including the yellow Roberto Clemente Bridge that's visible beyond the outfield fence of PNC Park.

Engineers have known since the 1970s that filling a hole with a weld — as opposed to a bolt, as is done today — can lead to weakness in a beam, but this kind of fracture was a surprise. Experts have said a rare confluence of events likely led to the break. Still, they said, identifying whether there are other plug welds in this bridge is prudent.

Inspectors are taking a close look at the major load-bearing beams on the bridge, seeking deformations in the smooth steel that could indicate other plug welds. They have finished reviewing the New Jersey half of the bridge and found a few such deformations that merit a closer look with technology similar to an ultrasound. A search on the Pennsylvania side is ongoing, but hampered by the repair efforts.

State officials are confronting the reality that this fracture likely happened quickly, but could have stemmed from an error that dates to the bridge's construction. It's a worst-case scenario, engineers said, and a constant worry in a state with one of the largest percentages of "structurally deficient" bridges nationwide, and some of the country's oldest infrastructure.

"That's always something that's in the back of my mind. We're the oldest interstate in America," Heigel said of the Pennsylvania Turnpike. "I'm concerned every day."