Two more arrests are expected in New York City, a source close to the investigation told CNN.
One of the individuals interviewed several times by authorities is cooperating, the source said.
Zazi's arrest comes after three days of questioning by the FBI. Zazi declined to attend a fourth day of interviews, his attorney's publicist said.
Zazi has admitted to having ties to al Qaeda, an administration official familiar with the matter told CNN on Friday.
The alleged terrorist plot, which came to light this week after raids in New York, may have been targeting a major transportation center, like a large railroad or subway station, sources close to the investigation told CNN on Thursday.
There were plans for an attack, presumably in the New York area, where crowds are large and security screening for nonairport travelers is lax, the sources said.
On Wednesday, federal agents searched Zazi's apartment in Denver and another home in a Denver suburb in connection with the investigation.
A law enforcement official told CNN that diagrams showing how to make bombs were found on the computer that Zazi had with him when he was stopped in New York during a recent visit
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