Authorities from both nations are trying to trace the movements of Faisal Shahzad during a recent five-month stay in Pakistan. They are seeking to establish whether he linked up with any of Pakistan’s myriad Islamist terror groups and received instructions, funding or training.
The 30-year-old Pakistani-American was arrested on Monday as he tried to leave the US on a jetliner headed to Dubai.
Shahzad, who lived in Pakistan until the age of 18, is accused of trying to detonate a crude gasoline-and-propane bomb inside an SUV in bustling Times Square on Saturday evening.
In recent days, authorities in Karachi have detained four alleged members of the Jaish-e-Mohammad militant group over their possible links to Shahzad, said two Pakistani security officials.
They said US law enforcement officials as well as Pakistani ones had been given access to the men.
Pakistan and the US are close allies and CIA officials have been known to question al-Qaida and other terror suspects there.
One of the officials said a detainee called Mohammad Rehan may have travelled with Shahzad to Peshawar, a main jumping off point to the Afghan border region.
Jaish-e-Mohammad is believed to have been established by Pakistani intelligence agencies, which used it to fight in Indian-ruled parts of the Kashmir region.
It has since been linked to al-Qaida. Its members and associates are suspected in the killing of Wall Street Journal reporter Daniel Pearl in Karachi in 2002, and in a bombing the same year in the city that killed 11 French engineers.
Shahzad has allegedly told US investigators that he spent time in Wazirstan, an al-Qaida and Pakistani Taliban stronghold.