One of the suspected planners of last month's attack by gunmen on Mumbai was arrested by Pakistani security forces in a raid on a militant camp, several sources in Muzaffarabad, the capital of Pakistani Kashmir, said on Monday.
An official with a charity linked to the militants said Zaki-ur-Rehman Lakhvi was taken into custody following Sunday's raid on a camp used by Lashkar-e-Tayyeba fighters, high in the hills outside Muzaffarabad.
"Yes, Lakhvi is among four or five people arrested in a raid yesterday," said the official, whose Jamaat-ud Dawa (JuD) charity is regarded as a front for the feared militant group.
A former militant who now has close ties to the JuD also said Lakhvi, who is one of Lashkar's operations chiefs, had been arrested, as did one intelligence official.
All of them were speaking on condition of anonymity because of the sensitivity of talking on security matters.
Other Pakistani intelligence officers said six men were arrested, but gave no names. There has been no official confirmation of the raid, let alone who has been arrested.
A British newspaper, the Times, in a report issued on its website on Monday also cited a government official as saying Lakhvi had been arrested.
Inevitably, there will be uncertainty until official confirmation is given, and a secretly detained person can always be secretly released.
Lakhvi was named as a ringleader in the Mumbai plot by the lone surviving gunman captured in India, according to Indian officials.
He and Yusuf Muzammil, the head of Lashkar's anti-India operations, gave orders by telephone to the 10 militants who killed at least 171 people in the attack on Mumbai, Indian officials say.
Pakistan has asked for proof that attackers came from Pakistan, while saying it will cooperate with India in the investigation, but tensions between the two nuclear armed rivals have risen.
The United States has exerted diplomatic pressure on Pakistan to match words with deeds swiftly to stop the crisis worsening, while asking India to exercise restraint.
"I think there's no doubt that Pakistani territory was used, by probably non-state actors," U.S. Secretary of State Condoleezza Rice told CNN's "Late Edition" on Sunday.
If Lakhvi's arrest is officially confirmed, it will raise the question of what the Pakistani authorities will do with him, and whether it will satisfy India.
President Asif Ali Zardari has said that anyone arrested in Pakistan will be tried there too.
The Pakistani military's Inter-Services Intelligence (ISI) agency had ties in the past with Lashkar and other jihadi organisations fighting Indian rule in Kashmir, according to analysts, which could reduce the Pakistani authorities' readiness to be transparent in its handling of the situation.
Laskhar was officially banned by Pakistan in 2001, after it was blamed along with Jaish-e-Mohammad for a raid on the Indian parliament that almost sparked a fourth war between the two countries.
The militants say Lashkar relocated its base to Indian Kashmir, while its founder Hafiz Saeed quit the organisation, but remained head of the charity.
Analysts say there is evidence of Lashkar fighters cooperating with al Qaeda, and there are fears that these jihadi organisations have become uncontrollable.
The JuD charity, which has thousands of followers, was also designated a militant organisation by the United States, but Pakistan has only put it on a watchlist.