One man was arrested on terrorism-related charges and another for possession of explosives, bringing to five the number of people arrested ahead of the Nato summit on Sunday in an alleged plot to target US President Barack re-election headquarters.
Chicago residents Sebastian Senakiewicz, 24, and Mark Neiweem, 28, were charged as part of the investigation that led to the arrest of three self-described anarchists on terrorism-related charges.
Senakiewicz was accused of falsely making a terrorist threat and Neiweem was charged with attempted possession of explosives or incendiary devices. Both will have a hearing to set bail on Sunday.
Senakiewicz' arrest came a day after prosecutors said three men were caught in the act of making Molotov cocktails, which they had discussed using against the Obama campaign headquarters, as well as Chicago Mayor Rahm Emanuel's home, several area financial institutions and four police stations.
Molotov cocktails are a crude bomb made of a bottle filled with a flammable liquid such as gasoline and usually fitted with a wick that is ignited and hurled.
Asked if the two Chicago men charged were related to the earlier arrests, authorities said they learned about them during the probe of the three others.
The three men charged previously were listed as Brian Church, 22, of Fort Lauderdale, Florida, Jared Chase, 27, of Keene, New Hampshire, and Brent Betterly, 24, from Massachusetts. Each is being held on $1.5 million bail.
The Chicago Police Department said those men, described as self-proclaimed anarchists and members of the "Black Bloc" movement that has disrupted international gatherings in the past, were charged with conspiracy to commit terrorism, providing material support for terrorism, and possession of an explosive incendiary device.
A lawyer for the three, Michael Deutsch, said undercover police officers had entrapped them by infiltrating the group and encouraging the bomb-making effort. Their supporters say the men had come to protest the Nato summit peacefully and that the police had confused beer-making equipment with explosives.
Word of the latest arrest comes as thousands of demonstrators are expected to march through Chicago Sunday as Obama hosts the heads of state of the Nato alliance for its two-day annual meeting. So far, two days of protests that began on Friday have been relatively peaceful, with few arrests.
Thousands of security personnel have been deployed to monitor them.