Detectives fear the prime suspect for the murder of Sabina Nessa is still at large, the Guardian understands, as the school teacher’s family spoke of their shock. Detectives are urgently seeking a man captured in CCTV images taken close to where Nessa, 28, was found dead on Sunday. Two men arrested in connection with the alleged murder have been released under further investigation. Nessa’s sister, Jebina Yasmin Islam, issued a statement on Friday evening before a rally at the East London Mosque – one of many vigils taking place across the country. “We as a family are shocked of the murder of our sister, daughter and aunty to my girls. There are no words to describe how we are feeling as a family at the moment,” she said. “We did not expect that something like [this] would ever happen to us. I urge everyone to walk on busy streets when walking home from work, school or a friend’s homes. Please keep safe.” She added: “I ask you to pray for our sister and make dua [supplication] for her. May Allah grant her paradise.” The man in the footage, who is seen dressed in casual clothing and apparently clutching a red or orange object in his right hand, was in the area of south-east London where Nessa was found dead on the evening she was attacked, police said. The killing has reignited concerns about the level of danger women face in Britain. A 12-second clip released on Friday by the Metropolitan police shows the balding man wearing a black hooded coat and grey jeans looking over his shoulder and pulling at his hood as he walks down a footpath. The Met police assistant commissioner Louisa Rolfe urged anyone who may know the man to contact them. Speaking outside the force’s New Scotland Yard headquarters on Friday, she said: “Anybody out there who may know who he is, or if you’ve seen that footage, and you have any information, no matter how small, please do get in touch. “This is an incredibly shocking and tragic incident. We’re working incredibly hard to secure justice for Sabina’s family who are at the forefront of our mind at the moment.” Police are investigating whether the attack started outside the block of flats where Nessa lived, with the victim forcibly taken to nearby Cator Park where she was killed, it is understood. Police declined to comment on reports in the Daily Telegraph that she was struck over the head shortly after leaving her block of flats, and then carried to the park. Rolfe said: “There will be lines of inquiry that we can share and those that we can’t, it’s really important that we maintain an open mind and do everything we can.” On Friday the Met said a 38-year-old man who had been arrested by homicide detectives on Thursday had been released under investigation. On Saturday, detectives arrested a man in his 40s on suspicion of murder before he was released under further investigation. Rolfe added she did not believe the two men who had been arrested in connection with the murder were known to each other. The Guardian understands the main and most urgent focus of the investigation is tracking down the man captured in CCTV images in Pegler Square near Cator Park on the night Nessa was attacked. Detectives have urged anyone who recognises the man, and a silver vehicle captured on CCTV in the same area, to contact them immediately. They are keeping an open mind as to whether the attack was by a stranger or someone who knew Nessa. Nessa, 28, is suspected to have been murdered as she made what should have been a five-minute journey on foot to a pub from her home, at about 8.30pm last Friday. Dozens more bouquets have been left in Cator Park where Nessa’s body was found. Tributes to the 28-year-old included a note reading: “Miss you so much. RIP my beautiful friend.” Another said: “She just wanted to meet her friend. She was minutes from her home. She didn’t deserve this.” Some police cordons have been removed from the north end of the park, including those that had sealed off the OneSpace community centre near to where Nessa’s body was found, though it remained deserted apart from a handful of officers patrolling on Friday.