Police believe a "highly professional soldier" could be responsible for a series of armed raids on homes in the Home Counties.
Meticulous research and the use of extreme violence by the lone raider suggest the thief, who uses a sawn-off shotgun, has military training, officials said.
One of his victims described how she feared she would be raped and killed during the raid on her Surrey home.
The burglar likely stakes out his targets for several weeks to track the movements of the wealthy occupants and learn the locations of safes and valuables inside.
The "well-spoken intelligent thief" is also believed to have entered some of the properties prior to the raids in order to better understand the layout of the rooms.
One victim, Susan Morris, suffered injuries, including a black eye, when her home in Kingswood, Surrey, was raided in October 2017.
The 61-year-old was home alone when the burglar, dressed in black and wearing a balaclava, crept up behind her before yelling: ""I want the jewellery, take me to the safe."
She continued: "He came towards me and I realised he had a sawn-off shotgun and he grabbed hold of me, he was holding me the whole time of the burglary and never let go of me."
After giving the thief her jewellery, Ms Morris said she was then told to "'get on the floor'... I actually thought he was going to rape me, I was very, very frightened".
During the raid, the burglar became angry, Ms Morris recalled, asking her for more and more jewellery, and then beginning to hit her.
"He hit me very hard on my face, not enough to knock me out, he knew exactly how hard to hit me, he hit me three times on my face, it was very painful, I couldn't believe the blows kept coming."
Ms Morris suffered a broken jaw, lost a tooth, and suffered bruising during the assault.
At one point during the raid, Ms Morris could not get a ring and bracelet off to give the the burglar, and she feared he would cut her finger off to get them.
Ms Morris described the burglar as "well-spoken", adding that she believed he had been "trained on how hard to hit someone to not knock them unconscious but to frighten them".
When Ms Morris' husband returned home, he found his wife hiding with her hands tied.
In another raid, the burglar was so confident that the alarm could not be raised that he spent more than an hour with the occupants until he had gathered all the items he wished to steal.
Detective Inspector Dee Fielding, from Surrey Police, said the use of cable ties, the level of planning and his confidence in carrying out such raids alone suggested he had a forces background.
"Each of these burglaries have been extremely distressing for the victims involved,” she said.
"We do not believe this to be the work of an opportunist burglar, but someone who has specialist knowledge and skills - possibly ex forces or from a similar background due to the way he has behaved and the circumstances of each incident.
"The burglaries all appear to be the work of someone who knows exactly what they are doing and who is incredibly decisive in their actions.
"On each occasion unnecessary levels of violence has been used in addition to a firearm, which he used to threaten the victims, who have then been tied up.
"He has worked quickly and appears to have knowledge of who is home at the time of each attack, the location of safes and the layout of the home.
"This leads us to believe he has planned the attacks in advance with a great level of detail and prior knowledge.
"More than £1 million worth of valuable items, including irreplaceable heirlooms, have been taken on each occasion."
Fielding added that DNA evidence had been found at the scene of one of the raids but this had not been traced to any other offences.
She continued that the raids have occurred about once every six months, possibly as the offender needed more money.
In a three-year period between November 2014 and October 2017, the thief, who has left little forensic evidence, has carried out seven raids - two in Kingswood, Surrey, and others in Maidenhead, Berkshire; Chichester, West Sussex, Sevenoaks, Kent; Maidstone, Kent, and Virginia Water, Surrey.