French Interior Minister Bernard Cazeneuve has said he will sue for libel after a senior police officer said she was harassed to change her report on the Nice terror attack.
The policewoman, Sandra Bertin, was in charge of security cameras.
She said she had been told to report the presence of national police units, although she had not seen them.
More than 80 people died when a lorry driven by Mohamed Lahouaiej-Bouhlel struck crowds celebrating Bastille Day on 14 July.
Ms Bertin told the Journal du Dimanche she had been instructed by an interior ministry official to report the presence of national police units on the seaside Promenade des Anglais at the time of the attack.
"The national police were perhaps there, but I couldn't see them on the video," Ms Bertin said.
She told the Journal du Dimanche she had also been "harassed for an hour" by Mr Cazeneuve on the phone.
The interior ministry said there were 64 members of the national police on the seafront as per arrangements with Nice city hall.
But the BBC reports Nice city hall believes some of these national police had been replaced by municipal police who were more lightly armed and less able to intervene to stop the killer.
The suspicion - denied vehemently in Paris - is that there has been a cover-up, the BBC says.
Mr Cazeneuve said he would sue for defamation over the "grave accusations".
The government has faced criticism for not preventing the attack at a time when France was in a state of emergency following the extremist attacks in Paris in November last year.
French ministers were booed when they attended a ceremony in Nice to remember the victims.
President Francois Hollande said on Friday he had "full confidence" in Mr Cazeneuve.
Last week, local authorities in Nice refused a request by French anti-terror police to destroy CCTV images of the attack.
The Paris prosecutor's office said the request had been made to avoid the "uncontrolled dissemination" of images.
But officials in Nice responded by filing a legal document, arguing the footage could constitute evidence.