Kenneth Williams, who has been on death row since 2000, was executed Thursday night, the fourth execution in eight days in Arkansas.

But activists say the rush to beat the expiration of a key lethal drug may have resulted in the death of an innocent man. On April 20, Ledell Lee was executed by lethal injection despite a flurry of last-minute legal moves to spare his life. Lee was convicted of murder in 1995 but always maintained his innocence. According to the ACLU, Lee was never given a fair trial due and may have been innocent. “[The death penalty] is really reserved for the people with the worst lawyers, and the poorest,” says Cassandra Stubs of the ACLU.