The NTSB team will take as long as it needs to gather evidence at the scene, Quimby said. Investigators typically spend five days to a week at an accident scene — even longer for something such as a major aviation disaster — and then take their investigation into the laboratories, he said.
Interested parties such as railroads, unions, manufacturers and regulatory agencies — in this case the Federal Railroad Administration — will be invited to participate.
Investigators then will begin the painstaking process of compiling the reports from the various team leaders into a draft finding of a probable cause that eventually must be approved by the agency's five-member board of presidential appointees.
The time it takes for the agency to reach its conclusions can be frustrating to those who want instant answers.
"Everybody wants to know yesterday what happened," Quimby said.
In another deadly transportation accident, the sinking of the water taxi Lady D in Baltimore harbor on March 6, 2004, it took two years for the NTSB to produce its final report, which placed much of the blame on the Coast Guard for the overloaded conditions that contributed to the boat's capsizing.
Others have taken longer, but those are relatively unusual.
"We generally say a year or 18 months," Weiss said.
Some retired NTSB employees say the agency is taking far longer than it once did to issue reports.
"When I left, it was frustrating to get the reports out because you had so many levels of reviews," Remines said.
Quimby, who participated in the investigation of several major train disasters in Maryland during his NTSB career, said he thinks the agency takes "an inordinate amount of time" to issue reports.
"Political sensitivity has become more and more involved, regardless of which party is in power," he said.
But the agency doesn't always wait for the final report to identify problems. Quimby said that if the NTSB discovers a condition that it believes poses an imminent danger, it will issue what is called an "urgent recommendation" without waiting for a final finding of probable cause.
That's what it did in the case of the Lady D, when it recommended in December 2004 — more than a year before its final report — that the Coast Guard revise its weight limit table for pontoon boats to more realistically reflect the weight of the average American.
In the Ellicott City investigation, it is far too early to know whether the agency will issue such recommendations. An early indication of what the agency is focusing on could come when the agency releases its "docket" in the case — a compilation of testimony, laboratory reports, maintenance records and other hard data that generally is made public before the agency announces any conclusions.
Quimby said that while the NTSB may be slow in making its final report, the agency is generally accurate.
"The safety board is generally pretty good at getting to root causes," he said.