Edx, although you could find it using the search funtion, I'll explain one more time why part serial number are generally collected by the during an ACCIDENT (key word there) Investigation.
The NTSB examines parts to determine if those parts might have been the cause or contributed to the accident. Some parts have serial numbers, some don't. Also, there are critical time change parts that airline maintenance are suppose to change on a schedule. Occasionally, the FAA or the AF (for AF Aircraft) mandates parts changes due to known problems. The FAA issues AD's while the AF issues TCTO for this.
Consequently, Accident Investigators need those serial numbers to determine if an AD or TCTO needs to be issued in case that part contributed to the accident. As well, they determine if the Airline or AF Maintenance changed the time change part on schedule.
Neither agency records part numbers to identify the aircraft as that is normally already known.
The aircraft involved in 9/11 were NOT ACCIDENTS, they were deliberate intentional crashes. Therefore, the FBI not the NTSB was the lead investigating agency. The investigation were NOT Accident Investigations, but Criminal Investigations. I don't think anyone knows if the FBI recorded any part numbers, but there seem to be none in the public record.
This is a common troofer "gotcha" as they know no part numbers are in the public domain. They then either imply or directly say "this indicates someone has something to hide" or it's evidence of "an inside jobby job".
This stuff originated with George Nelson, Col, USAF Retired who issued a statement to include much of what I've stated above. However, he embellished the story with "trutherisms" to imply that it was very suspicious that these part serial numbers were not recorded indicating nefarious action.
Nelson opinion is in part due to his Vietnam experience in which some aircraft were only identified by part serial numbers. These would include those aircraft not in formation, with a single pilot, or a case in which all crew members were killed. Frequently, there was no other way to identify the aircraft because most of the flying was in a non-radar environment, most AF combat aircraft do not have an FDR on board, and DNA identification did not exist.
Your best argument is to demand the Regulation/Instruction that specifies that part numbers are specifically used to prove aircraft identity or that these serial numbers in any way are required to identify a crashed aircraft. Troofer opinions don't count to include George Nelson's implications.