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."
More top news
Dry for most of tonight, with clear spells at first
The officer tried to hold off the group at Finsbury Park - where Drake and Mary J Blige were headlining.
The image, taken at the weekend, shows the man making his way past Big Ben with a girl perched on his shoulders waving an ISIS flag.