A forensic pathologist has told a court that a stab wound to the back caused the death of 15-year-old Luis Guembes, as experts gave evidence about DNA findings linked to two teenagers accused of his murder.

Dr Anna Biddlestone, who conducted a post-mortem examination at Royal Surrey County Hospital on January 21, told Guildford Crown Court that Luis died from a stab wound to the left side of his lower back.

The wound, estimated to be 17cm deep, had damaged his kidney and major blood vessels, causing severe internal bleeding.

Dr Biddlestone said the injury would have caused rapid blood loss, although the full extent of the bleeding might not have been apparent during the post-mortem examination.

She also identified wounds to Luis' head and shoulder but said these were less serious and would not have contributed significantly to his death.

The court heard that two knives had been examined during the investigation.

An ornamental knife recovered from Defendant 1's home had a double-edged blade measuring approximately 26cm, while a second knife found outside Defendant 2's address had a blade around 11cm long.

Dr Biddlestone said a knife with a blade measuring approximately 14–15cm would have been a more plausible candidate for the fatal injury.

She could not entirely rule out either weapon but considered the ornamental knife unlikely because of the width of its blade.

The pathologist also described superficial scratches on Luis' body which were consistent with contact with vegetation.

Bruising to his hand could have resulted from a struggle, while another minor injury might have been defensive, although she acknowledged other possible explanations.

Forensic scientist Claire Morse then gave evidence about DNA and bloodstaining found on clothing, footwear and other items recovered during the investigation.

Bloodstains on Defendant 2's footwear, hat and grey clothing produced DNA profiles matching Luis.

Ms Morse said the findings could be explained by Defendant 2 having been close to Luis while he was bleeding.

Blood matching Luis was also found on a soap dish recovered from Defendant 2's bathroom.

A small bloodstain on a mop matched Defendant 2, although Ms Morse said there were several possible explanations for this.

At Defendant 1's address, no bloodstaining was detected on an ornamental knife or several items of clothing examined by investigators.

DNA recovered from the knife's cutting edge produced a mixed profile, with Defendant 1 among the more prominent contributors.

However, Ms Morse stressed that the absence of blood did not establish that a knife had not been used, as blood might not have transferred or could have been removed.

She also explained that DNA evidence could not establish when biological material had been deposited or precisely how it had been transferred.

The forensic findings alone could not determine whether either defendant had been involved in the assault, the court heard.

Both defendants, who are now 16, deny murdering Luis, who was a pupil at All Hallows Catholic School in Farnham.

The trial continues.