Separately, the Iraqi authorities also announced arrests of three other "terrorists", two of whom were caught disguised as women.
The US military said a massive manhunt by Iraqi and US forces led to the arrest last month of Mohammed Hilal Hammad Ubaydi, also known as Abu Ayman, a former aide to the intelligence chief under Saddam Hussein.
It said he had "strong ties" to al-Qaeda's Iraq frontman, Abu Musab al-Zarqawi, and was the leader of the Secret Islamic Army, a militant outfit operating in the Babel province south of Baghdad.
He was caught on March 7 in Al-Mahmudiyah, south of Baghdad, but his arrest was not immediately announced pending a DNA test, which "confirmed this is Abu Ayman," the military said.
The military said he was the prime suspect in the kidnapping of Sgrena and was also behind a series of assassination attempts on Iraqi government and security officials.
The Sgrena kidnapping last year drew worldwide attention when US soldiers shot dead an Italian agent shepherding her to safety after she was freed.
Insurgents seized Sgrena from outside a Baghdad mosque on February 4, 2005, and numerous uncorroborated claims emerged for her kidnapping.
But later in a video, on which "Mujahideen Without Borders" was digitally stamped, she was seen pleading with Italy to end the US-led occupation of Iraq.
After a month in captivity Sgrena was released amid allegations that Italy had paid a hefty ransom to secure her freedom.
However, while on her way to the Baghdad airport she and the Italian agents escorting her came under fire from US soldiers at a checkpoint. One agent was killed and two others wounded.