(CNN) -- Investigators are focusing on about a dozen more people in connection with a wide-ranging terror investigation that has netted arrests in Colorado and New York City, a source familiar with the investigation said Tuesday.
A lawyer for Najibullah Zazi disputes claims that bomb-making plans were found on his client's computer.
Federal agents arrested three men late Saturday in connection with what the Justice Department has said was a plot to detonate bombs in the United States.
Najibullah Zazi, his father Mohammed Wali Zazi and Muslim cleric Ahmad Wais Afzali are charged with lying to federal agents.
All three suspects are originally from Afghanistan. Mohammed Wali Zazi is a naturalized U.S. citizen, while Afzali and Najibullah Zazi are permanent legal residents. If convicted, each could face eight years in prison.
A federal magistrate set bail at $50,000 Monday during the men's first appearances in courtrooms in New York and Colorado.
The elder Zazi, 53, whose son is at the center of the investigation, will be allowed to go home under electronic monitoring and required to remain there except for work, medical care, religious services or court appearances, prosecutors said. He is unlikely to be released until later this week.
Prosecutors are asking that his son, 24-year-old Najibullah Zazi, be held until trial. He is scheduled to attend a detention hearing Thursday. Watch why authorities wanted to talk with Zazi »
Investigators said Najibullah Zazi has admitted attending courses and receiving instructions on weapons and explosives at an al Qaeda training facility in Pakistan's tribal areas during a 2008 trip.
They also say he lied to federal agents about explosives-handling instructions they found on his computer.
His attorney, Arthur Folsom, has denied the allegations.
Afzali appeared in a federal court in New York, where a not-guilty plea was entered on his behalf. He smiled as he listened to the magistrate read the charge against him, blowing kisses to his wife during the hearing.
Ron Kuby, Afzali's lawyer, said his client had tried to help federal agents find Najibullah Zazi and called the charge against his client "a bootstrap case" the government brought "to cover up their own failings and the fact that they were the ones who blew this investigation."
According to affidavits outlining the charges against him, Afzali warned Najibullah Zazi that his phone call was being monitored. But Kuby said Afzali was helping "absolutely frantic" federal agents find Zazi, who had attended his Queens mosque with his family as a teenager.
"My client didn't tip him off, he was already tipped off," Kuby said. "And now, the FBI is looking for somebody to blame."
The plot may have been targeting a major transportation center, such as a large railroad or subway station, sources close to the investigation said last week.
CNN's Kathleen Johnston, Jeanne Meserve, Jim Spellman and Deborah Feyerick contributed to this report.