According to the boy's account, Ana left the area ahead of him, which was the last time he saw her.
He told gardai that as he went to leave the area, he was assaulted by two men and was injured. Gardai are following up the boy's account and are trying to track down the two men he says attacked him. They are also examining blood found on the boy's clothing.
Ana Kriegel's body was found in the derelict farm, outside Lucan, Dublin, at lunchtime on Thursday. She left her family on Monday at around 5pm to go to St Catherine's Park and was reported missing by her parents three hours later.
Gardai believe that Ana was murdered on Monday evening. They suspect that she was killed by someone that she knew and that she may have been sexually assaulted.
Preliminary post mortem results suggest she was beaten and died of blunt-force injuries. Detectives are believed to have recovered items of Ana's clothing along with blood-spattered sticks and bricks, which they hope will provide them with crucial DNA evidence.
Results of preliminary DNA tests are due back tomorrow.
The boys are among several people who knew Ana who are speaking to gardai about her final moments.
Gardai have been reaching out to her friends through their families, looking for their help in order to piece together her movements and who she was in contact with in her final days and hours.
Gardai are also examining Ana's mobile phone and social media accounts.
The brutality of the crime has shocked the local community, many of whom laid flowers at the entrance to the derelict farm where her body was found.
Friends paid tribute to Ana as a tall and striking girl who loved dancing and swimming. She was a first year student at a local secondary school.
The scene at the farm remained cordoned off this weekend as members of the Garda Technical Bureau arrived to carry out further examinations.
Gardai have appealed to others who were in touch with Ana or might have some information about her plans for Monday evening to contact them at the investigation headquarters in Leixlip at 01 666 7800 or on the confidential number 1800 666111.