Spainish police stand guard at the main entrance of the Torrejon de Ardoz air force base on Thursday after Spain's security forces found a "suspect" package. Photo: AFP/VNA

The package was intercepted at the US Embassy by security officials and was later detonated in a controlled explosion by Spanish police.

The bombing campaign began with a package sent to Spanish PM Pedro Sanchez last Thursday, spurring Madrid to tighten security around public buildings. Similar devices were also sent to the defense ministry, the Torrejon de Ardoz air force base, the Zaragoza arms factory, and the Ukrainian embassy, where a security officer was slightly injured.