The suspect in thethat left eight people dead and another 12 injured left a note that referenced the Islamic State of Iraq and Syria ( ), a federal law enforcement source confirmed to CBS News.
CBS News has learned, 29, is the suspect in the attack, which New York City officials describe as an act of terror.
Sources tell CBS News' senior investigative producer Pat Milton that Saipov yelled, "Allahu Akbar" -- Arabic for "God is great" -- when he exited a rented Home Depot pickup truck after hitting pedestrians.
Saipov came to the U.S. from Uzebekistan in 2010, CBS News has confirmed.
says there was an extremist group there, called the Islamic Movement of Uzbekistan, which pledged allegiance in 2015 to ISIS after about seven years of al Qaeda affiliation. Both al Qaeda and ISIS have urged followers to carry out vehicle attacks.
According to Townsend, investigators will be looking into whether he was motivated by IMU or other extremist groups and if he had any contact with him -- which is one reason they will be searching Saipov's electronic devices.
New York Governor Andrew Cuomo said the attack appeared to be a "lone wolf" incident with no apparent link to a.
Saipov doesn't appear to have an extensive criminal background, CBS News' justice and homeland security correspondent Jeff Pegues reports. Both the FBI and the NYPD are "digging" into Saipov's background, Pegues reports.
An Uber spokesperson confirmed to CBS News that Saipov was a driver for them and had passed a background check. He has been banned from the app. Uber has been in contact with the FBI and law enforcement.
NYPD Commissioner Joseph O'Neill said Saipov drove onto a bicycle path at Houston and West Street at 3:05 p.m. local time. The suspect struck several pedestrians and cyclists before colliding with a school bus at Chambers St., O'Neill said. After exiting the vehicle and yelling, he was shot by police and taken into custody.
Cuomo, O'Neill and de Blasio praised the work of an officer, identified by CBS New York as 1st Precinct Officer Ryan Nash, a five-year NYPD veteran, who they said stopped the assailant before more people were hurt.
CBS New York said Nash was taken to the Bellvue hospital for ringing in his ears following the shooting. He is a native of Long Island.