Santa Maria del Popolo
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.
Tiberio Crispo
As papal legate to Perugia, Crispo was a "driving force behind the architectural renewal of the city". For example, in 1547 Crispo commissioned Galeazzo Alessi for the construction of Santa Maria del Popolo to replace a church demolished by the construction of the Via Nuova. He also commissioned a palace in Bolsena that bears his name, "Palazzo di Tiberio Crispo" (also known as the "Palazzo Crispo Marsciano" or "Palazzo Rondanini alla Rotonda"), which was designed by Antonio da Sangallo the Younger circa 1543; after the death of Sangallo in 1546, Raffaello da Montelupo was called in to finish the palace, which remained incomplete, however, after the deaths of both Crispo and Raffaello in 1566.