After the $N$-particle generalization of the Veneziano amplitude was written down and studied, who was the first (or who were among the first) to realize that the amplitudes could be understood in terms of a relativistic string in the early 1970s?
Veneziano writes in Ref.1:
[...] Inded these last three hints were not missed, and the proposal that a string was lying at the basis of all of those magic properties that have been found was finally made, particularly, by Nambu [Nam70], Nielsen [Nie70] and Susskind [Sus70]. The identification remained quantitative for some time until Nambu [Nam70b] and Goto [Got71] first formulated in a precise way the classical action of a relativistic string, and then the work of Goddard, Goldstone, Rebbi and Thorn [GGRT73] established the connection between the dual resonance model (DRM) spectrum and that of a quantized string.[...]
- G. Veneziano, Rise and fall of the hadronic string. Published in The Birth of String Theory (Eds. Cappelli, Castellani, Colomo, Di Vecchia), Cambridge University Press, 2012.