Molecular scan is a BAD idea
Several people in other threads, have asked, why the security forces didn't think to use the molecular scanner to determine if a suspected imposter is human or not. Well given the ending of the film I think the answer is obvious.
If you have a previous scan to compare the new scan to and it proves your suspect is an innocent human after all you've saved one life. But if the scan proves the suspect is an imposter thousands of people will lose their lives when the imposter self destructs.
The reward, saving one life, doesn't away the risk, losing many lives.
Same reason the scan had to be interrupted when the suspect tried to scan himself. It it proved he was an imposter, the veterans hospital would have been vaporized.