Aged 16, he returned to the Sammarinese Musical Institute to study composition, but was left in a month-long coma after a traffic collision, which caused him to lose his memory.
[1] Messieri later enrolled at the Conservatorio Giovanni Battista Martini in Bologna, where he was a contemporary of Alessandro Solbiati, and studied under Giacomo Manzoni, Franco Donatoni, Aldo Clementi, Ivan Fedele, Niccolò Castiglioni, Gérard Grisey, and Lelio Camilleri.
Camilleri inspired him to attend IRCAM's Academie d’Eté in Paris between 1993 and 1994, where Messieri was a contemporary of Tristan Murail and Philippe Manoury.
[1] In 2002, Messieri returned to San Marino to teach at the Sammarinese Musical Institute, where he met saxophonist Michele Selva, with whom he formed a duo that debuted in Dresden and Prague, releasing two compact discs.
In 2018 the Mascagni Conservatory of Music in Livorno dedicates him a concert for the "Suoni Inauditi" International Festival, commissioning him Salmo XLIII, lyric for soprano, male chorus, baritone saxophone and live electronics (Elena Tereshchenko, soprano; Valerio Barbieri, saxophone; Maestro Gabriele Micheli, conductor); in the same year Nicola Baroni commissioned him BET (Concerto grosso for soprano, 4 cellos and violoncello orchestra) for the Modern Festival of the Teatro Comunale in Bologna.
In 2020 during the Covid pandemic, Messieri writes a series of compositions for solo piano, and a year later they are recorded on "Islands" Cd by Patrizia Romanello for the Amadeus Arte Label.