Details about the six were released late Friday, though officials have not said when they were detained. Five were picked up in the capital, Islamabad, and one is co-owner of a posh catering company that the U.S. Embassy said was suspected of ties to terrorist groups.
The intelligence official, part of the team questioning the men, cited the two suspects as saying they did not do anything wrong and "proudly" describing Shahzad as their friend.
The official said one of the suspects had even accused his interrogators of "siding with the infidels."
One of the suspects, identified as Shoaib Mughal, is alleged to be a go-between for Shahzad and Pakistani Taliban in their hide-outs close the Afghan border. He was running a large computer dealership in Islamabad before his detention, said the intelligence official who - like most operatives in spy agencies around the world - did not give his name.
The other suspect, identified only by his first name Shahid, is alleged to have helped arrange money for Shahzad. He has an MBA from the U.S. and apparently knew Shahzad from his time there.
The other four suspects have also expressed their hatred for the West and the U.S., but have not admitted any links with Shahzad, the official said.
None of the men has been charged, though in Pakistan that sometimes does not happen for months, if not years, particularly if detainees are held by an intelligence agency.
Pakistani intelligence cooperates closely with the CIA, which is often given access to detainees.
Shahzad is accused of leaving an SUV rigged with a homemade car bomb in New York's Times Square on May 1 that failed to explode.
The 30-year-old was born in Pakistan and moved to the United States when he was 18. The son of a former air force officer, he led a privileged life. He has family roots in the northwestern city of Peshawar and grew up in at least one other city, Karachi, relatives and officials have said.
Shahzad claimed during a long interrogation that he received financial support from the Pakistani Taliban, two U.S. law enforcement officials close to the probe said Friday, speaking on condition of anonymity because the investigation has not been completed.
Investigators believe his U.S. funding was channeled through an underground money transfer network known as "hawala," the officials said. But, one official said "there's a belief that no one in the U.S. who got him the funds was aware of what they were for."
He was the only person in the United States who was "operational" in the plot, the official said.
Among those detained in Pakistan is Salman Ashraf Khan, the co-owner of the upscale Hanif Rajput Catering Service. Two other suspects "wanted him to help bomb a big gathering of foreigners" whose event his company was catering, the Pakistani intelligence officer said.
Khan's father said Saturday he was baffled by the accusations because his son is a successful businessman who lived happily as a student in the U.S. for four years. The younger Khan studied hotel management in Florida and computer science in Houston, returning to Pakistan in 2001 to take over the family business.
"How can a man who is so much involved in this business be accused for such an activity, which only a wild animal can think about?" Rana Ashraf Khan said in a telephone interview.
"He might have differences about whatever has been going on in our region for the last 10 or 11 years, we all have differences," Khan said. "(But he had) no feelings against the United States at all. He lived there happily, he studied there."
The family reported Salman Ashraf Khan missing after he failed to turn up at his office on May 10. A neighbor's guard saw a man drop off Khan's car at the family home about an hour after he left for work, then speed off in a taxi with several others, the father said.
Hanif Rajput Catering Service is popular among foreign embassies and many of Pakistan's wealthiest companies and individuals. The U.S. Embassy warned Friday that the catering company was suspected of ties to terrorist groups and said American diplomats had been instructed to stop using the firm.
Also detained was a former major who bought his way out of the army because of a "disagreement with its policies," said the intelligence official.
The ex-major is from Rawalpindi, where the army headquarters is situated. Last week, an army spokesman denied anyone connected to the army was arrested in the probe, saying only a retired major had been arrested on disciplinary grounds and was being investigated.
The link to the army is noteworthy because of the Pakistani military's past support for Islamist militants in Afghanistan and Kashmir, and Shazad's family ties to the air force. It was unclear whether the suspect's alleged ties to Shahzad were ongoing when he still served.