Doing some thread necromancy here
I'm probably wrong. The odds against suicide are pretty high. He was a Catholic and they had all those other kids to support. But maybe that's part of it, maybe he worked himself to exhaustion. The family was left destitute after the search, they couldn't even afford to bury her. Even with all the donations coming in, it must have been pretty hard.
They weren't Catholic, and from what info I can find it seems like the date of Frank's death was a coincidence (or perhaps indirectly caused by the stress).
Also, I'm going to straight up repudiate some of the earlier posts in this thread, particularly by jerseylily who has since gone inactive it seems- the anti-Rromani (g*psy) speculation has no basis in reality. The whole idea of Rromani people kidnapping children, particularly "blue-eyed, yellow-haired children" to quote the investigators at the time, is on the level of blood libel in terms of dangerous and untrue. Frank's insistence that the "g*psies" had taken his daughter was more based on a) cultural attitudes- both what he likely brought with him from Europe (specifically, the area that is now the Czech Republic) where anti-Rromani attitudes are unfortunately still quite prevalent and the general view of Rromani as inherently criminal, deceptive, and threatening to 'white' people that was commonplace at the time (not to say that America doesn't have anti-Rromani ideas and stereotypes today- it's more that there's a distance to it because there isn't a huge population that many Americans interact with and thus base their attitudes off of pop culture rather than actual malicious intent); and b) a lack of knowledge that could be perhaps called a degree of "innocence" or naïveté. Child murders, particularly 'stranger' non-familial murders and sexual abuse, were not something that 'happened' (obviously they did, but it wasn't something that was accepted as 'happening', so to speak, compared to today where we know and acknowledge that this happens). Nowadays we can understand the psychology and potential inner workings of such a murderer; at the time it was likely harder to think about someone, perhaps a neighbor or passer-by, being the murderer than a member of an 'out' group, like the Rromani people. Overall, it seems that there is no evidence to actually point towards the "g*psies", and in fact the single-minded focus on them may have caused the investigators to miss clues or suspects.
I personally think that it was perhaps someone, local but perhaps from the next neighborhood over who Elsie may have vaguely recognized. He likely lured Elsie in, took her to his apartment where he would have lived alone without much interaction with his neighbors, and assaulted her. Her death may have been an accident from the killer trying to stop her from crying out, or intentional.
