Who is being referred to by the statement "Then your hearts became hardened" in the āyah?

Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. All the Children of Israel.
2. The killer(s). Ibn ʿAbbās said: Those who killed him, after the murderer was named, said: "By Allah, we did not kill him."