The gun was not "tampered with". A few parts broke during testing and the broken parts were replaced to conduct further testing.
Alec lied when he told law enforcement and others he did not pull the trigger. He pointed the gun at a woman, pulled the trigger and shot her, killing her.
"GUN ANALYSIS
Experts in ballistics and forensic testing based in Arizona and New Mexico relied on replacement parts to reassemble the gun fired by Baldwin — after parts of the pistol were broken during earlier testing by the FBI. Their report examined the gun and markings it left on a spent cartridge to conclude that the trigger had to have been pulled or depressed.
The analysis led by Lucien Haag of Forensic Science Services in Arizona stated that although Baldwin repeatedly denied pulling the trigger, “given the tests, findings and observations reported here, the trigger had to be pulled or depressed sufficiently to release the fully cocked or retracted hammer of the evidence revolver.”
An earlier FBI report on the agency’s analysis of the gun found that, as is common with firearms of that design, it could go off without pulling the trigger if force was applied to an uncocked hammer — such as by dropping the weapon.
The only way the testers could get it to fire was by striking the gun with a mallet while the hammer was down and resting on the cartridge, or by pulling the trigger while it was fully cocked. The gun eventually broke during testing."
From:
What to know about grand jury evidence on actor Alec Baldwin and the 2021 fatal film set shooting | Fox 59