(CNN) -- The Department of Homeland Security and FBI have issued security bulletins to raise awareness regarding "terrorist interest" in attacking sports and entertainment venues as well as luxury hotels.
The bulletins, which were sent to law enforcement Monday, said that authorities did not know of any credible or specific terrorist plots to attack U.S. stadiums, arenas or luxury hotels.
However, it said that terrorist groups such as al Qaeda view crowded stadiums and arenas as potential targets. It said hotels are also attractive targets for terrorists.
The Department of Homeland Security said it released the notes to assist law enforcement partners as they go about their daily duties.
Meanwhile, investigators are looking for about a dozen more people in connection with a wide-ranging terror investigation in the United States, and more arrests are expected, a source familiar with the ongoing investigation told CNN Tuesday.
Three men were arrested late Saturday in connection with what the Justice Department has said was a plot to detonate bombs in the United States. They are charged with lying to federal agents during the probe of the alleged plot.
The men made their initial appearances Monday in courtrooms in New York and Colorado.
Prosecutors asked that Najibullah Zazi, 24, of Denver, Colorado, be held until trial. Zazi is at the center of the investigation. A detention hearing was set for Thursday.
Investigators said 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. He is accused of lying to federal agents about explosives-handling instructions they found on his computer. His attorney, Arthur Folsom, has denied the allegations.
A federal magistrate in Denver set bond at $50,000 for Mohammed Wali Zazi, 53, Zazi's father. He 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. Authorities said he is unlikely to be released until later this week.
The Zazis face charges along with Ahmad Wais Afzali, a 37-year-old Muslim cleric and funeral director from the New York borough of Queens. Afzali appeared in a federal court in New York earlier Monday, 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."
Afzali was ordered to remain in custody until a bond hearing later this week.
Kuby said his client had been an occasional source for New York's Joint Terrorism Task Force when it looked into previous terror plots, and a detective who had sought out Afzali as a source in previous cases would testify on the imam's behalf Thursday.
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 would face eight years in prison.
The plot may have been targeting a major transportation center, such as a large railroad or subway station, sources close to the investigation told CNN last week. But federal agents have "no specific information regarding the timing, location or target of any planned attack," David Kris, assistant attorney general for national security, said in a written statement over the weekend.
The complaint against Afzali says he falsely asserted that he never told Zazi that agents were monitoring him on the phone -- an assertion Kuby disputed.
"Why on Earth is the imam going to lie to the FBI about the contents of a conversation that he knows they're recording?" Kuby told reporters.
The criminal complaint against Afzali also says he falsely told federal agents that he did not ask Zazi about evidence in his rental car.
The probe emerged after a series of raids in Queens last Monday, three days after the intercepted calls. A police stop of Najibullah Zazi during a visit to New York had raised investigators' concern that he would discover he was under surveillance, a former counterterrorism official familiar with the investigation told CNN last week.
At the Queens residence where Zazi stayed during his visit, FBI agents seized a black scale containing several AA batteries. Zazi's fingerprints were on both, the criminal complaint against him said. And agents searched his rental car and laptop computer while he was there, turning up what they said were handwritten notes of his that explained how to make, handle and detonate explosives, according to the complaint.
An FBI expert determined the handwriting in the photographed document was similar to Najibullah Zazi's, according to the complaint. But during an interview with investigators Wednesday, Zazi denied having the instructions and said that if it was found on his computer, he must have unintentionally downloaded it as part of a religious book. He said he deleted the book within days after he realized that its contents discussed a holy war, investigators said.
According to court records, Mohammed Wali Zazi falsely told investigators that he had not called, nor had he received a call, from anyone in New York asking him about his son's activities. He also said he did not know anyone named Afzali despite recorded phone calls that showed he did, the FBI stated.