Mullan’s story crumbled a few days later when he admitted that he didn’t even know about the light collapse until a month after it happened. As word spread that Mullan’s job might be in jeopardy, Mullan forced someone else out instead. Not Ernst, his former first deputy, but Frank Tramontozzi, who was Ernst’s supervisor while the state highway administrator was on family leave. Mullan concluded that Tramontozzi, not Ernst, was most responsible for leaving him in the dark on an important safety issue.

Despite these private doubts, Mullan told reporters he and his staff had “spent quite some time’’ investigating the incident and that he was disclosing it after getting “a better idea of exactly what we were dealing with.’’ He said they had deliberately kept the incident quiet to avoid panic while they worked.

“I have to ask what we have been doing for six weeks. No testing yet?’’ lamented Mullan in an e-mail hours before a March 16 press conference where he belatedly told the public about the light fixture hazard in the tunnels.

State records also show that his agency’s attempt to solve the problem was both more secretive and sluggish than he admitted. Engineers, led by Mullan’s close associate Helmut Ernst, didn’t even send the fallen fixture to a lab for analysis until March 16, instead leaving the crucial piece of evidence in a South Boston maintenance facility along with mounds of road debris.

Internal e-mails and Transportation Department reports obtained by the Globe show that last winter’s light fixture collapse presented a more hazardous situation than Secretary Jeffrey B. Mullan disclosed to the public, and one that could add $200 million to the already-gargantuan price of the Big Dig.

But the engineers in charge kept quiet. They filed no written report. They didn’t brief their boss. And when they asked federal regulators for money to fix a corrosion problem that “could’’ lead to falling light fixtures, they didn’t disclose that one had already fallen.

The Big Dig already had a tragic experience with dangerous falling objects. A tunnel ceiling panel had collapsed in 2006, killing a woman a few hundred yards from where the nine corroded fixtures were discovered on Feb. 16. State engineers had no way of knowing how many more of the 25,000 lights in the Big Dig tunnels had become unstable - and plenty of reason to fear that corrosion was widespread after years of saltwater leaking into the tunnels.

A few days after a 110-pound light fixture crashed from the ceiling of the Tip O’Neill tunnel onto one of the busiest roads in Boston, a highway crew made an alarming discovery: nine other lights in the sprawling Big Dig tunnels were hanging from supports so corroded that they could fail at any time, too.

“He told me, ‘Frank, it is either you or me and I’m not going,’ ’’ said Tramontozzi, still bitter at being forced out just a few weeks before he became eligible for a state pension. “Either you resign or you’ll be fired.’’

Ernst, who received a letter of reprimand in the incident, said Tramontozzi didn’t deserve to be fired, and he questions Mullan’s current claim that he didn’t know about the corrosion problem for a month.

“I have a hard time believing [Mullan] didn’t know,’’ said Ernst, chief engineer of the region that includes the Big Dig. “I wish he had just stuck with the first [story] and been done with it . . . You don’t push your problems down to the employee below you because your job is at risk.’’

Mullan declined to discuss details of Tramontozzi’s departure in an interview on Friday but said, “I made it clear [to Tramontozzi] that I had lost confidence in the leadership of the highway division.’’

Mullan, who has previously admitted he “mismanaged’’ public disclosure of the fallen light fixture, also said he never believed his job was in jeopardy.

The light fixture fiasco underscores a culture of secrecy among the engineers who maintain the $22 billion Big Dig, one of the world’s most costly - and problem-plagued - highway projects. After years of scandals, bad press, and lawsuits alleging shoddy work, engineers freely admit they don’t like to write things down, prompting one transportation consultant to compare the atmosphere to President Nixon’s White House.

The consultant, C. David Taugher, wrote in a March internal report, “How deep does the culture go where nobody says anything, even when they know they should?’’

Five months after the collapsed light, the Transportation Department still isn’t sure why hundreds of light fixtures showed significant premature corrosion, making them vulnerable to collapse. Mullan has said that he believes road salt from the highway got into the lights, but lab analysis showed the cause could just as easily be salt from the ocean water that constantly leaks into the tunnel, costing the state $12 million a year to keep out and generating headlines in 2004 when it flooded the roadways. Ernst calls the cause of the corrosion problem a “mystery.’’

Whatever the cause, Ernst said last week that all 25,000 lights ultimately have to be replaced and the tunnels rewired, a project that could cost $200 million. For now, crews are securing the lights with plastic straps that they began installing three weeks ago.