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A spokesman for the Metropolitan Police said:
"All lines of inquiry were considered in the early stages of the investigation. The investigation was detailed and complex and officers had a duty to explore every avenue of inquiry and retain an open mind about the circumstances.
"They carried out numerous actions including door-to-door inquiries, leaflet drops, searches, interviews and forensic analyses.
"The CCTV evidence took some time to collate and analyse but then clearly showed a veiled suspect trailing the victim before the attack.
"Konye was arrested as the investigation progressed."
Ms Oni said officers were incompetent "to kind of accuse me of doing it to myself, when I've said somebody has followed me from my workplace to my home".
"I even got told that they watched the CCTV footage and they saw no-one following me," she said.
"We've all seen the CCTV footage. There was a person in a veil following me.
"Why was this not found in the beginning? Nobody has answers to tell me."
Naomi Oni, a Victoria's Secret shop assistant, was left scarred for life after suffering serious burns on her face and chest in the incident in Dagenham, east London, in 2012.
Her former friend, Mary Konye, 22, who disguised herself in a Muslim veil before throwing the sulphuric acid, was jailed for 12 years last week.
Ms Oni told the BBC Radio 4 Today programme that police were slow to act on information she gave them about the attack.
She said it almost felt like she had "been accused of lying", adding that she had suffered "pain and backlash for her (Konye's) actions and other people's incompetency".