Researchers in Italy say they may have found traces of a Leonardo Da Vinci work hidden under a Florentine fresco.
Tiny probes, sent through drilled holes in Giorgio Vasari's The Battle of Marciano in the Palazzo Vecchio, found black pigment also used in the Mona Lisa, project workers claimed.
"These data are very encouraging," said the project's leader Maurizio Seracini.
But historians at a press conference in Florence stressed their research was "not conclusive".
They added that further chemical analysis needed to be carried out.
"Although we are still in the preliminary stages of the research and there is still a lot of work to be done to solve this mystery, the evidence does suggest that we are searching in the right place," said Seracini, who works at the University of California in San Diego.
The probes also discovered red lacquer and brown pigment on the hidden wall.
The research has been controversial, with some art experts signing a petition to stop the investigation because the drilling is damaging Vasari's existing work.
Tomaso Montanari, an art historian who has led the opposition to the research said that he did not "consider the source of these findings credible."
He added: "What do they mean by saying the findings are compatible with Leonardo? Any painting from the Renaissance would be. Anything from that era could be painted on that wall."
"What lacked here is a neutral team that has the scientific authority to evaluate this. It is very complex."
Seracini believes Leonardo's unfinished The Battle of Anghiari lies beneath Vasari's work.
It is believed the artist started painting his fresco - which is considered by some to be his finest work - in 1504 but abandoned the project because of problems arising from his experimental oil painting technique.
The room was later renovated and Vasari painted his fresco in 1563.
Seracini believes Vasari did not want to destroy Leonardo's work and instead bricked it up behind a new wall on which he painted.
His theory was stimulated after finding a soldier on Vasari's work holding a small flag bearing the words: "He who seeks, finds."