Logs at the army headquarters showed the bomber entered the military compound and staged the attack last month after posing as a soldier's pregnant wife to take advantage of "maternity day" at the army hospital.
There were local media reports that the bomber was indeed pregnant.
But police spokesman Rienzie Perera told reporters that authorities were still carrying out forensic tests on blood samples taken from parts of the woman, who was blown to pieces after the April 25 attack.
"The story that she was pregnant is only hearsay," Perera said.
"We still don't know if she was really pregnant."
Investigators said pregnancy tests by local laboratories proved negative and detectives were now trying to secure the help of forensic experts abroad.
Investigators initially believed she was a 21-year-old woman from the northern district of Vavuniya, but that too proved to be wrong, an investigator said.
Police said her mobile phone was also destroyed but they managed to recover the SIM car