Fibres linked to Stephen Lawrence found by a cold case squad on suspects' clothes were not discovered earlier because it was like "looking for a needle in a haystack", a court has heard.
Angela Gallop, who examined exhibits linked to the murder in 1995 and again in 2007, told jurors at the Old Bailey that the later search was much more comprehensive.
The earlier tests involved looking at 1,071 fibres while the examination 12 years later by private firm LGC found around 4,500.
She said: "It's a bit like looking for a needle in a haystack and some needles are easier to find than others. We didn't find them and we didn't look at nearly as many fibres as LGC did."
Dr Gallop added: "Perhaps if we'd gone on we might have found them, I don't know."
Gary Dobson, 36, and David Norris, 35, deny taking part in the gang attack that killed Mr Lawrence in Eltham, south-east London, in 1993.
The prosecution argues that tiny amounts of fibres, blood and hair found on clothes seized from their houses prove they were involved in the murder.
The defence says that the samples got onto the clothes through contamination during handling and storage.
Dr Gallop was instructed as part of an unsuccessful private prosecution brought by Mr Lawrence's parents and first examined the exhibits in 1995.
Twelve years later she was working for LGC, which was instructed to carry out a cold case review of Mr Lawrence's murder.