Police say they wanted dental checks on two killers of Zainab Kalokoh, who was shot dead at a christening, because they believe they lied about their age.
The brothers were born in Nigeria and may be deported
Diamond Babamuboni and his brother Timy say they are 17 and 15 respectively - but police believe they are older.
They wanted the south London pair to undergo dental checks to prove their age, but they refused. Neither police nor courts can enforce checks.
The pair, with Jude Odigie, 16, will be sentenced for manslaughter in February.
Born in Nigeria
"Officers were not convinced those were the ages the defendants claimed to be," a Scotland Yard spokesperson said.
If police were able to prove they were older than they claimed, it means the pair would be face harsher penalties as they would no longer be tried as minors.
Police cannot be sure of their ages because the pair were not born in the UK, but in Nigeria.
Dental checks can establish an approximate age by checking wear on teeth.
Zainab Kalokoh, 33, was shot in the head when robbers raided the party in Peckham, south London, in August 2005.
A 17-year-old, who cannot be named for legal reasons, was found guilty at the Old Bailey of murdering Mrs Kalokoh.
An order preventing the identification of the defendants was lifted for all but the murderer.
Brian Altman, prosecuting, said Mrs Kalokoh, who had fled war-torn Sierra Leone, had only recently arrived at the party to celebrate the christening of her niece, Adama Yalie.
Mr Altman said the gang, armed with a sawn-off shotgun, a pistol and several knives, burst in through a fire exit shortly after 2200 BST.
They fired into the ceiling and the prosecution said a handgun was then discharged by another into Mrs Kalokoh's head.
Zainab Kalokoh was shot in the head as she held a baby
Mr Altman said she fell to the floor, still clutching Adama, who "incredibly" was unharmed.
He said the gang, all of whom lived in Peckham, and who were wearing balaclavas, then ordered some of the 100 party guests to lie on the floor while they robbed them of their valuables.
The four were also convicted of robbery and possession of a firearm at the time of committing an offence.
At the time of the murder the families of the Babamuboni brothers and Odigie were seeking refugee status but the Home Office has since turned down their application and they will be deported once their sentences have been served.