The announcement of the arrest of Zarrar Shah - following Sunday's detention of another alleged Mumbai plotter, Zaki-ur-Rehman Lakhvi, plus 20 other alleged militants - could deflect intense U.S. and Indian pressure on Pakistan following the attack.
But much will now depend on whether Pakistan's relatively new civilian government keeps up the pressure on the militant groups that are believed to have been fostered by the country's powerful military and intelligence agencies.
Pakistan has targeted militants in the past, detaining some - only to quietly let them go later, bolstering critics who claim Islamabad is not serious about fighting extremists.
Pakistani officials insist that its old foe, New Delhi, has not shared any evidence with them that links the suspects to the attacks, raising questions as to how the country can bring them to trial. Islamabad has already said it will not hand them over to India.
Last month's attacks on Mumbai, India's financial center, killed 171 people and sharply raised tensions between the nuclear-armed neighbors, which have fought three wars since 1947. India says all the attackers were Pakistani citizens.
Pakistani Prime Minister Yousuf Raza Gilani gave no details on Shah's arrest except to say that he and Lakhvi "were in (Pakistani) custody and were being investigated."
Both men are alleged to be members of Lashkar-e-Taiba, a banned Pakistani militant group blamed for other attacks on Indian soil and with historical ties to the country's shadowy intelligence agencies.
Indian media reports citing intelligence officials have identified Zarar Shah as Lashkar's communications chief and said he created the communications system that allowed Lashkar leaders in Pakistan to stay in touch with the gunmen during the siege.
The New York Times has reported the attackers and their handlers used Voice over Internet Protocol (VoIP) phone services for communication to make it more difficult for investigators to trace their calls.
The paper has also quoted American and Indian officials as saying that Zarrar Shah was one of Lashkar's primary liaisons with Pakistani intelligence. It did not elaborate, but U.S. officials have said there is no evidence linking the attacks to any Pakistani state agencies.
Unnamed Pakistani officials have previously said Lakhvi was detained Sunday in a raid on a Lashkar camp close to the Indian border that netted several other extremists.
Indian investigators say the sole Mumbai attacker captured alive has told them that Lakhvi recruited him for the assaults. They also have said the assailants called another senior leader, Yusuf Muzammil, on a satellite phone before the attacks. Muzammil's whereabouts are not known.
Gilani said India had shared no evidence or information with Pakistan about their suspicions surrounding the men.
"We are investigating on our own about the people they have identified (through the media)," he said in the central Pakistani town of Multan. "That is a good message to our neighbors and rest of the world that Pakistan is a responsible nation."
New Delhi has so far not commented on the arrests in Pakistan.
On Tuesday, investigators in Mumbai released the names, photos and hometowns in Pakistan of the nine militants they said carried out the three-day siege.
Gilani and other officials declined to comment on that development.
Pakistan's intelligence agencies and military have long used militant groups as proxy armies, both in Afghanistan and against Indian troops in the disputed region of Kashmir, the trigger for two of their wars.
Ayesha Siddiqa, a Pakistani defense analyst, described the two arrests as "a minor first step which the government has taken as a gesture."
After a 2001 attack on the Indian parliament by alleged Pakistani militants, Islamabad arrested the two leaders of the country's main pro-Kashmir militant groups. They were released without charge less than a year later.
Siddiqa said the civilian government may be not be able to crack down on the militants entirely because of pressure from the military, elements of which still regard India - not the militants fighting it - as the country's main enemy.
"It may not be completely in control of things," she said.
(This version CORRECTS ADDS Giliani and other offficials declining comment on release of name of 9 alleged attackers; corrects spelling of Shah graf 6; minor EDITS)