Rosenker's letter was a response to Oberstar's complaint that Rosenker compromised the objectivity of the NTSB investigation.
Earlier this month, Rosenker issued a status report on the investigation, saying the cause of the Aug. 1 bridge collapse was poorly designed gusset plates, and that corrosion wasn't a factor.
Oberstar wrote to Rosenker last week, criticizing him for basically jumping the gun on announcing a cause of the collapse, noting that the NTSB investigation is still going on.
"This means that the Board has not yet determined the role played by the poor design of the gusset plates in causing the accident," said Oberstar in the letter. "The Board has not yet determined whether the design of the plates was the primary cause of the accident, compared to other possible causes such as corrosion or poor maintenance."
"In these circumstances I consider it highly inappropriate for you to have stated, as you did in the press conference, that the poor design of the plates 'tells us why the bridge collapsed,'" Oberstar continued. "Although you admitted that you did not hve complete information on corrosion, you nonetheless insisted that poor design was the 'critical factor.'"
Investigators haven't actually done any analysis on the bridge collapse yet, and the NTSB isn't expected to complete the investigation for another six months.
In his letter Monday, Rosenker wrote that he did not intend to cite a cause for the collapse. He also backed away from his comment that corrosion was not a cause.
Rosenker said he meant only that the under-designed gusset plates didn't show signs of corrosion. He also emphasized that the circumstances that led to the bridge collapse were still under investigation.
Rosenker wrote that he appreciated Oberstar's support of the NTSB.