Three robbers have struck three times in three weeks in Hove and Portslade.
The same trio are believed to be behind another robbery – in Shoreham – in the middle of last week.
Sussex Police said that two 17-year-old boys from Hove had been arrested on suspicion of robbery.
They have been bailed until January while detectives continue to investigate.
The first hold up happened at Martin’s newsagent in Station Road, Portslade, as the shop was opening at 6am on Friday 28 November.
Three people came into the shop and told the shop assistant to turn off the lights.
They grabbed her by the neck and forced her into the backroom where they made her open the safe.
The three robbers took £1,600 cash as well as cigarettes worth about £3,000. They escaped through the rear fire exit which leads into St Aubyns Road.
The shop assistant was shaken but unhurt.
The other robberies happened as businesses closed for the day.
Ten days ago, at 10.55pm on Monday 8 December, the Tates Texaco garage in Old Shoreham Road, at Southern Cross, was robbed as a member of staff was locking the door.
Three men pushed into the garage, stole several hundreds of pounds from the safe and took cigarettes.
The staff member saw a knife and was punched in the head but was not seriously injured.
Two days later, at about 11pm on Wednesday 10 December, the One Stop shop in Buckingham Road, Shoreham, was robbed.
As the two employees were closing the door they were forced back into the shop and ordered to hand over cash. Several hundreds of pounds were stolen.
Both employees were threatened and one was struck in the face. She was not seriously hurt but extremely shaken.
The latest robbery was at Unwins in Queen’s Parade in Hangleton Road, Hove, close to the Grenadier pub, at 10.40pm on Tuesday (16 December).
The staff member had been at the counter, preparing to close the shop, when two men came in, approached him and demanded the keys. He handed them over and they locked the door.
One of the men forced him around the back of the shop, leaving the other man at the till.
He was punched in the face before being robbed of his wallet, mobile phone and his own keys.
He was then forced to open the safe from where money was taken. The robbers also took cigarettes before leaving in the Applesham Avenue direction.
Detective Chief Inspector Paul Betts said: “We are linking all four of these robberies due to the way these men go about the offence and taking cash and cigarettes.
“Their descriptions are also similar with the victims describing them as two black men and a white man all dressed in dark clothing and balaclavas.
“We are asking shops in the Portslade and Hangleton areas especially to be vigilant and aware of these robberies when closing up for the night and to consider changing their routines regularly.
“We are asking anyone who has been offered cheaper cigarettes or seen anyone selling a large amount which could be suspicious to get in touch.
“Anyone who has seen anything suspicious around these areas at the time of the robberies or has seen anyone acting strangely, please contact police.”
Anyone with information is asked to phone Sussex Police on 101, email email@example.com or call the independent charity Crimestoppers anonymously on 0800 555111.