Police did not immediately say if the explosion appeared to be linked to the four others. The first three were parcel bombs dropped off in front of homes on Austin’s east side, with the fourth an apparent trip wire device that went off on the city’s west side on Sunday.The four devices were similar in construction, suggesting they were the work of the same bomb maker, officials said.FedEx officials could not be reached immediately comment.

The first two bombs killed black men and investigators believed that the third, which injured a Latina woman, may have been intended for a black family’s home, police said, raising the possibility they were a hate crime.

Sunday’s tripwire bomb, which injured two white men, went off shortly after police made a rare public call to the suspect to explain his motives.