The Vatican's Secretary of State signed the letter, which was addressed to the Archbishop of Mechelen-Brussels, Jozef De Kesel, on behalf of the Pope.
He said that Francis expresses "deep sympathy for the wounded, for their families and for all those contributing to relief efforts," and prayed that the Lord would bring them comfort and consolation.
Pope Francis, he said, "again condemns the blind violence which has caused so much suffering, and he implores God for the gift of peace, invoking upon the grieving families and on all Belgians the benefit of divine blessings."
The Pope's prayers come after at least 34 people were killed and 170 more injured in March 22 attacks at Brussels Zaventem international airport and a city metro station near buildings belonging to the E.U.
Twin blasts hit the airport around 8 a.m. local time, tearing through the departure section. The BBC reports that a Belgian prosecutor said the blasts were likely caused by "a suicide bomber."