WASHINGTON — While federal investigators are months away from a final decision, internal memos suggest they have settled on undersize steel plates and heavy loads of construction materials as the likely cause of the deadly bridge collapse in Minneapolis last year.

The National Transportation Safety Board memos point to gusset plates, used to hold the bridge’s girders together, that were too small and more than 191 tons of rock and sand dumped on the bridge for a repaving project.

The memos also show the deliberations among NTSB staffers and board members over whether to hold an interim public hearing or wait until the board takes up the final report in a meeting later this year, as the board ultimately decided.

NTSB Managing Director Joseph Osterman wrote in one memo that “the outcome of our investigation appears to be clear, so showing our cards at a public hearing or in the final report is simply a matter of timing.”

In January, NTSB Chairman Mark Rosenker said at a press briefing that the undersizing of the gusset plates was “the critical factor” in the 40-year-old bridge’s collapse on Aug. 1, which killed 13 and injured 145. Rosenker did stress that a final cause had not yet been determined.

Greg Feith, a former safety investigator for the NTSB, said that the board in some cases has “locked on” to a cause.

“The objective of the NTSB is to remain objective until all facts and circumstances have been developed,” he said.

In this case, Feith said, “apparently, they’ve been able to garner enough information that they believe that the combination of those two factors is what led to the loss of the bridge. That’s apparently where the investigation will come to fruition.”

The board will ultimately deliberate on the final staff report, and it’s rare when the board votes not to approve any of the findings and tells the staff to do it over, Feith said. But the board members may have questions about issues that are not adequately addressed in the report, he said.

NTSB spokesman Terry Williams, asked how people can expect that a final decision hasn’t already been reached, said that a probable cause won’t be determined until the board meeting later this year.

“That’s the actual deliberation,” he said. “And this has not come before the board yet. The actual deliberations can be pretty engaging. We do not have a probable cause. We have not had any deliberation. It’s still an ongoing investigation.”

While the NTSB has decided not to hold a public hearing on the investigation before the final report, the investigation itself will be a subject of a public hearing in the House Transportation Committee.

The committee chairman, Minnesota Democrat Jim Oberstar, who has been highly critical of Rosenker, will have pointed questions for the NTSB, said Oberstar spokesman John Schadl.

The committee would have conducted the hearings anyway as part of the NTSB reauthorization. But Schadl says the hearings will now have a specific focus on the investigative process.

“Jim is concerned that any investigation by the NTSB be perceived as independent and credible,” he said. “This is the agency that sets the gold standard for independent and nonpartisan investigations.”

A negative reaction in Congress to not hold a public hearing was anticipated by Osterman, who wrote in one memo that some “political and public interest groups and individuals” wanted one.

If the board votes not to hold a hearing, he wrote, “I am convinced that these groups will be motivated to become more vocal and critical of the board’s decision and may actively work against our efforts during reauthorization and appropriations. Although unfortunate, I believe these folks could harm our efforts.”