Police were questioning a man today in connection with the shooting of five-year-old Thusha Kamaleswaran.
The 19-year-old was detained in south London yesterday following the shooting on Tuesday in which the little girl and another innocent bystander were caught in the crossfire of a suspected botched turf war attack inside a shop.
Scotland Yard said the man was being questioned at a south London police station.
A Metropolitan Police spokesman said yesterday: "Trident detectives investigating a shooting in Stockwell that left a five-year-old girl and a 35-year-old man critically injured have arrested a man this afternoon, Thursday March 31.
"The 19-year-old man was arrested in south London and is currently in custody at a south London police station whilst inquiries continue."
The arrest came after a teenager targeted by gangsters who accidentally shot Thusha came out of hiding to help detectives find the gun attackers.
Relatives are maintaining vigils at Thusha's hospital bedside while detectives build up a "continuity trail" of the attackers' movements.
Police said the child remained in a "critical but stable" condition.
Meanwhile, beat police are beefing up their presence around Stockwell Food and Wine store as sources said up to 50 gangs were operating in the south London area.
Extra police support - including several specialist Territorial Support Group officers - have been called in to provide help to reassure locals that there are no "tit for tat" retaliation attacks.
Close family friend Velluppillai Navaratnam, 49, from Croydon, south London, said it was a "very difficult time" for Thusha's relatives.
"The parents of Thusha are at the hospital now," he said. "We are all praying for her to get well soon."
Thusha - believed to be London's youngest gun crime victim - was hit in the chest and Roshan Selvakumar, 35, suffered a head wound as two black youths cowered in the store after being chased by three other black youths on bicycles. Police said Mr Selvakumar's condition was stable.
Detectives revealed Thusha's 12-year-old brother, three-year-old sister and mother had been in the shop but were unhurt during the shooting. The store's owners are the children's aunt and uncle.
Crimestoppers is offering a £50,000 reward for information leading to the gun attackers' capture, it was also announced.
Detective Chief Inspector Tony Boughton told reporters at the scene that officers "are beginning to get a clearer picture" after one of two black youths who ran into the shop to flee the attack came forward to assist with inquiries.
"We are still trying to trace the other individual and appeal to him to come forward," he said.
Initial forensic reports support witness statements that two shots were fired. Evidence is also being analysed in a bid to work out if the attackers were linked with previous crimes.
"There's a lot of CCTV in the area and we are trying to follow a continuity trail to find the youths responsible," Mr Boughton said.
He said they had already had a good response from the community in their appeal for information.
The three attackers, who a witness said were between 14 and 17, fled from the Stockwell Road shop along Broomgrove Road and into the Stockwell Park Estate.
The victims, both Sri Lankan and unrelated, were taken to hospital in south London.
"Although their injuries are very serious, they are both under the circumstances doing well," Detective Chief Inspector Tony Boughton said.
Officers are still painstakingly searching through CCTV in a bid to track down the culprits.
Chief Superintendent Nick Ephgrave called for people in the area to come forward if they have information about the "truly shocking event".
Mr Ephgrave, the borough commander, said he had requested extra support from Scotland Yard to reassure the community.
"You should see in the next few days some reassurance that this is an isolated incident," he said.