Armed robberies have taken place in four Northampton stores and one in Daventry store in the space of two weeks.
The early-morning raids have prompted detectives to urge shopkeepers to review security arrangements.
The Northampton offences took place on September 22 in Limehurst Square, Duston, at about 6.30am, September 26 in Bushland Road, Headlands, at about 7.15am, September 27 in Kent Road, Duston, at about 6.45am and September 28 in Newnham Road, St David's at about 6am.
The Daventry robbery happened on September 30 in Wimborne Place at about 5.50am.
A police spokeswoman said: "Detectives are following a number of lines of inquiry and believe it is likely some of the offences are linked.
"However, they are keeping an open mind and are urging anyone with any information about those involved in these offences, to contact police as soon as possible."
The force is also encouraging owners, managers and staff to be extra vigilant and to review the security measures they have in place, to ensure they minimise the opportunities for robbers who might look to target their premises.
Detective Inspector Dave Harley, who is leading the investigation, said: “We are investigating a series of robberies at local convenience stores where armed offenders have threatened staff and forced them to hand over cash from the tills.
"The offences have happened around 6am to 7am while staff have been opening up.
“We are doing all we can to find the people responsible for these offences, including examining CCTV footage, conducting forensic tests and speaking to witnesses whenever possible. I would urge anyone with any information about those involved these crimes, to contact us as soon as possible.
“Our crime prevention officers are able to give advice to the businesses affected to make sure they have as much security in place as possible, and we are encouraging owners, managers and staff to follow as many of our crime prevention tips as possible.
“I would also appeal to staff and customers to remain vigilant and please call us if you see anyone acting suspiciously near a retail premises. Call 101 to report any suspicious activity, or 999 if you believe a crime is taking place.”
Businesses may be targets for robbery due to a range of factors, including the goods they provide or sell, location and opening hours, but there are measures that can be put in place to reduce the risk to the business and staff.
Below are some simple precautions and more detailed information can be found in Northamptonshire Police's downloadable advice guide: Protecting your business against robbery:
- Keep windows clear. Remove advertising or posters from windows if they obstruct the view of staff and any passers-by
- Don’t hold large amounts of cash in your till. Impose a realistic till limit that you and your staff stick to
- Don’t count cash in public view. Cashing up should take place in a back room, preferably where the safe is located, with the door locked
- Staff should be extra vigilant at opening time and in the lead up to closing time
- Staff should be trained in how to deal with the public in violent or confrontational situations. They should also be trained in the firm’s procedures, security measures and what to do in the event of a robbery
- Keep a log of all suspicious incidents, including date, time, description of suspicious people or vehicles and what they were doing. Report such activity to the police on 101
- Advertise your security measures with posters and signs in prominent locations. If a robbery does take place, remain calm – try not to panic and don’t make sudden movements
- Think about your safety and that of your customers and staff. No-one should risk their life by trying to stop or detain an offender
- If there is a hold-up/panic alarm installed then use it, but only if safe to do so
- Try to remember as much detail as possible about what happened and the suspects and call the police on 999 as soon as possible, giving as much information as you can to the call operative
Anyone with any information is asked to call Northamptonshire Police on 101. Alternatively, they can call the independent charity Crimestoppers anonymously on 0800 555 111 or use the anonymous online form at www.crimestoppers-uk.org.