Pakistani police have arrested three men in connection with the kidnapping of 70-year-old American Warren Weinstein more than 10 days ago.
Police officers declined to give details about who the men are and what their connection to the Aug. 13 abduction may be. The three were detained by police Tuesday but were not officially arrested until today, authorities said.
No group has come forward to claim the kidnapping or issue demands in exchange for Weinstein's return -- an unusual development which sparked concern among some officials over Weinstein's fate. But one Pakistani intelligence official told ABC News last week there was no evidence to suggest the abductors had intended to kill Weinstein or that he had been accidentally killed.
"The Pakistanis are leading a very vigorous investigation," U.S. Department of State spokesperson Victoria Nuland said today, noting that the FBI and the U.S. embassy are assisting in efforts to track Weinstein down.
READ: American Mystery in Pakistan: Cops Release Kidnapping Suspect Sketch
Weinstein, a private U.S. citizen who has lived in Pakistan for seven years, was sleeping in his bed when assailants burst into his home to snatch him. The former USAID worker is currently employed by the private U.S.-based development firm J.E. Austin Associates.
All three suspects are from the same province in which Weinstein lived, an area far from the turbulent tribal regions near the Afghan border more usually associated with violent attacks. The men were arrested after investigators managed to track their cell phone numbers, the Lahore police chief said without elaborating.
Weinstein's friends and colleagues describe him as a diligent worker dedicated to helping Pakistani people.
"He is a tireless worker for development in Pakistan," Geoff Quartermaine Bastin, who worked with Weinstein after meeting him six years ago, told ABC News. Bastin said Weinstein "worked 18-hour days, three phones at once while talking to a fourth person at the table."
"He is very smart, very motivated and loved Pakistan and its people. He is careless of his health and safety, going everywhere to push his projects," Bastin said.
Weinstein suffers from a heart condition, and J.E. Austin Associates released a long list of medications that he takes, appealing to his abductors to provide them for him.
Some in Pakistan have speculated privately that Weinstein was not a development worker, but instead worked in intelligence for the U.S. Punjab Law Minister Rana Sanaullah publicly announced his suspicions last week, telling local media that Weinstein was involved in "quite suspect" intelligence-gathering for the U.S. government and compared him to Raymond Davis, the American CIA contractor who was jailed in Pakistan earlier this year for shooting two men on the streets of Lahore.
U.S. diplomats said Weinstein is not connected to any U.S. intelligence groups.
Weinstein is the first private citizen to be kidnapped in Pakistan since al Qaeda operatives abducted and murdered Wall Street Journal reporter Daniel Pearl in 2002.
ABC News' Lee Ferran and Kirit Radia contributed to this report.