The supermarket giant was forced to close the stores on Saturday. Although Tesco did not give a reason for the closure, reports suggest the security alert could have been triggered by a bomb scare.
Saturday was a national day against Tesco by animal rights groups, but Hertfordshire police, which is leading the investigation, said it did not believe the incidents were linked to extremism.
Further reports suggest a possible financial motive behind the threats. Tesco is estimated to have lost more than £2m from the closures.
“Inquiries to identify those involved in a series of threats to 14 Tesco stores on Saturday continue,” a police spokeswoman said. “Each of the stores involved has been searched and given the all clear and the company continues to work with the constabulary.”