When the lights dimmed
In 'Reb Moshe: The Life and Ideals of HaGaon Rabbi Moshe Feinsten,' (New York: 1986) by R. Shimon Finkelman the following appears on page 258:
In the beis midrash at the Yeshiva of Staten Island, boys and young men were fervently reciting tehillim, imploring Hashem to spare him. At 9:50 p.m. the light fixture over his seat near the Aron HaKodesh flickered and went out. Then it glowed for a moment and went out again.A reliable person told me the following:
Reb Moshe was gone and the light went dark.
A[nother reliable person] was a bachur in MTJ at that time. The Mashgiach called him out of the Tehillim group and told him that Rav Moshe had just passed on and that he should go to the basement and pull the fuse that powered the lamp over the shtender (which apparently was on a different fuse than the lights in the Beis Medrash) as a way to break the news to the Talmidim gently. [He] did so.