Armed police are patrolling the streets of Hull until a serial bank robber is caught.
The move follows two robberies in the city in two days.
In both cases, the man passed a note to cashiers saying he had a gun and demanded money.
Police have increased patrols across the city and have alerted every bank, warning them to tighten security. The Mail understands armed officers have also been deployed in the streets.
Detective Chief Inspector Paul Cunningham said: "We have put a lot of extra police in the streets around the city to make sure we are covered as well as we can be to provide reassurance and respond if we need to.
"Although the threats so far have been made purely to bank staff, we can't rule out that this person may become desperate and make other threats.
"We are treating this very seriously and, although no weapon has been seen, we have to work as though there is a weapon. We have a lot of officers involved in the investigation and a lot out on the streets.
"Although we recognise there is a raised threat level at the minute, I would like to think people will carry on as normal but remain vigilant."
The robber first struck at Yorkshire Bank in Princes Avenue, west Hull, at just before 1pm on Tuesday.
The following day, he targeted NatWest in Hessle Road, west Hull, at 2.45pm.
Both were busy with customers at the time. During the two offences, the man told staff he had a gun, although no weapon was seen.
He escaped with a large amount of cash from both banks.
Det Chief Insp Cunningham said: "These are traumatic events which have a lasting effect on people. Even though they have not seen a weapon, they have felt under threat, which is why they have felt forced to hand over money.
"No firearm has been seen, but we can't rule out that he is carrying one. We are working as quickly as we can to identify this person to let people go back to normality.
"As a result of this, banks are adopting additional security measures. We have circulated the information to all banks, so they are aware of what has happened."
During both robberies, the man joined a queue of customers before passing a note to the cashier when he reached the counter.
Both notes said he was armed with a gun and demanded money.
Det Chief Insp Cunningham said: "There are obvious similarities between the two robberies. Although we are keeping an open mind, there are enough similarities to investigate them together.
"The suspect is relatively calm and he seems quite brash and confident about not being caught.
"Although he has taken some precautions to cover his face, he is committing these offences in busy streets, and at busy times of day. There appears to be an element of planning of things such as escape routes."
The Humberside Police helicopter was used to search for the man after the offence at NatWest. The offender is believed to have run down Constable Street after the robbery.
Witnesses have provided differing descriptions from the two offences but police still believe the same person could be responsible.
Officers said the cash stolen on Wednesday may be stained with pink dye, which could have transferred to the offenders' clothing.
They are appealing for people who see the suspect not to approach him and to call police immediately.
Det Chief Insp Cunningham said: "If people are using banks, I would tell them to take care and remain vigilant.
"These offences are very rare in Hull, but we have now had two similar robberies in two days.
"We need to identify this person as soon as possible to prevent any further offences."
After the first robbery, the suspect was described as being a white man, aged between 30 to 40 years old, about 5ft 10in and of slim build.
He was wearing a woollen hat, which covered part of his face, dark blue tracksuit bottoms and white trainers.
The suspect in the second robbery was described as being a white man, aged between 20 and 30 years of age, wearing glasses and with facial hair. He was wearing black track suit bottoms, a black hooded Adidas tracksuit top with red stripes over the shoulder and head.
Anyone with information about either robbery should call Humberside Police on 101 or Crimestoppers anonymously on 0800 555111.
