Rome, Italy (CNN) -- The oldest known image of the apostles Andrew and John have been discovered in catacombs under the city of Rome, dating back to the 4th century AD, archaeologists announced Tuesday.
The paintings were found in the same location where the oldest known painting of St. Paul was discovered last year, the Pontifical Commission for Sacred Archeology said Tuesday.
They are part of a group of paintings around an image of Jesus as the Good Shepherd on the ceiling of what is thought to have been a Roman noblewoman's tomb, experts said.
A painting of St. Peter makes up the fourth member of the group, but older images of him are thought to exist, Vatican experts said.
Their inclusion in the tomb shows the aristocrats were among the last Romans to convert to Christianity, archaeologist Fabrizio Bisconti said.
The Roman matron must have been very rich, he said, as the colors and richness of the decoration show.
The images of the apostles' heads and shoulders against a deep red background were uncovered after two years of work, Vatican experts said.
Archaeologists used a new laser technology to remove layers of white carbon calcium deposited on the frescoes over the centuries without disturbing the paintings.
They are located in the catacombs of St. Tecla, one of the 40 Roman catacombs under Rome. It sits under a modern eight-story building in a working-class neighborhood. It is closed to the public and its entrance is mostly hidden.
The Vatican spent about 60,000 euros (about $74,000) on the archaeological work, it said.
The apostles were a group of a dozen men, according to Christian tradition, who spread the gospel of Jesus after his crucifixion.
CNN's Hada Messia contributed to this report.