The mistrial was announced in the downtown Los Angeles courtroom where the trial of Stephen Beal was being held.
Federal prosecutors say they plan to retry the case, which is scheduled to return to court Oct. 18.
The 2018 explosion destroyed the Magyar Kozmetica spa and left its owner dead.
Prosecutors said the motive was a crime of passion. The spa owner, Ildiko Krajnyak, had previously broken up with Beal.
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The prosecution said Beal was obsessed and sent numerous emails, poems, letters and cards to Karjnyak. They said her friends had warned her that he was controlling and possessive.
Prosecutors also said Beal not only knew Krajnyak but also had access to the spa and had the specific skill set to be able to make a bomb. His hobby was model rockets.
In his home, investigators found 130 pounds of pyrotechnic chemicals.