In the United States, police in Austin, Texas, have said four explosions that rocked the city, killing two people, are believed to be the work of a serial bomber.
They say the latest explosion on Sunday night, which injured two white men, aged 22 and 23, may have been activated by a tripwire.
Three parcel bombs left on doorsteps have killed two people and injured two others since early March.
Investigators say they have seen similarities in all four cases.
The two men killed earlier this month were African Americans and police have not ruled out racism as a possible motive.