The architect or architects of this innovative project remain unknown due to a lack of contemporary sources. Giorgio Vasari in his "Lives" attributed all the important papal projects in Rome during Sixtus IV to a Florentine, Baccio Pontelli including the basilica and monastery of Santa Maria del Popolo. Modern researchers deemed this claim highly dubious and proposed other names among them Andrea Bregno, a Lombard sculptor and architect whose workshop certainly received important commissions in the basilica. The fundamental differences between the façade and the interior suggest that maybe more than one architect was working on the building.