The move was not made in haste. Marco spent many minutes considering his move, before he made this fatal error.

Here is the position at that moment:

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Emil Kemeny in "the American Chess Weekly" noted that the losing move was not made in haste. Kemeny continued, <Marco after some deliberation was about to capture the Bishop, when he withdrew his hand. Pillsbury got up and informed some of the players that his opponent came near making a blunder. Perhaps ten minutes later the contemplated play was actually carried out.>

Kememy explained that Marco had evidently calculated that White would reply to 37...Bxf5 with 38.exf5, for if 38.Qxf5, Black would get out of danger by Kg8. Fritz indicates that 38.Qxf5 Qe7 or 38.Qxf5 Rf8 are equal. After 38.Qxf5 Kg8, White has a very small edge: (.27) (22 ply) 38...Kg8 39.e5 Ne3 40.Qh7+ Kf8 41.Bg6.

If White replies to 37...Bxf5 with 38.exf5, Kemeny indicated Black could play 38...Bxd4, threatening Qh7+ and Rg8+ winning the Queen, if White captured the Bishop by 39.cxd4.

However, Fritz shows that Kemeny's suggested line 37...Bxf5 38.exf5 Bxd4, will lose to 39.f6!, and it is mate in three!

Best after 37...Bxf5 38.exf5 is 38...Qe7, but White is still winning after 39.Qh5+ Bh6 40.f6 Qxf6 41.Qxe8+ Bf8 42.Rg3.

Kemeny indicated the correct move for Black at move 37 was 37...Re6, with a probable draw.

Fritz agrees that 37...Re6 was best: (.40) (20 ply) 38.Qh4+ Kg8 39.e5 Kf8 40.Nxg7 Qxg7 41.Bf5 Bxf5 42.Rxg7 Kxg7 43.Qg5+ Bg6 44.b3 Nxe5 45.dxe5, with a likely draw.