The second suspect in the Woolwich murder was known for handing out radical leaflets, and a woman believed to be his girlfriend had recently converted to Islam.
Other details known about the man, said to be Michael Adebowale, are that he is 22 and that he frequented the area of southeast London where Thursday's atrocity took place.
He is in hospital after being shot by armed response officers but his injuries are not life-threatening, a Scotland Yard spokesman said.
The man is six years younger than his fellow suspect and in video clips captured by passers-by his behaviour seemed less volatile.
He had been seen frequently by neighbours at a block of flats in nearby Greenwich, which was raided by armed police yesterday.
Police wearing balaclavas and bulletproof vests and armed with submachine guns smashed their way into the flat, thought to be occupied by two sisters.
Two children and a baby wrapped in a duvet were taken away, witnesses said.
Neighbour Nicola James, 45, said of the family: "When you walked past you would hear them singing Muslim songs. You would hear them chanting through the windows. The gran who used to live there died and since then there has been a lot of comings and goings. I saw the man who was in the video go inside the house. I think he was a boyfriend.
"The woman I think he went out with wore a full niqab. She converted to Islam about 2 years ago."
Another witness said the man was known for handing out radical leaflets.
Police removed a laptop computer and two bags of items from the house.