The brother of Asian terror suspect Hambali is in Pakistani custody, the authorities in Islamabad have confirmed.
Rusman Gunawan was among a number of foreign students arrested in Karachi at the weekend.
"Hambali's brother was under surveillance for weeks," Interior Minister Faisal Saleh Hayat said on Tuesday.
Hambali is alleged to have links to al-Qaeda
Hambali is accused of being the operations chief for Jemaah Islamiah (JI) - a group allegedly linked to al-Qaeda and blamed for last year's Bali bombing and other attacks.
He was arrested in Thailand in August and is currently in US custody at an undisclosed location.
Hambali's real name is Riduan Isamuddin - but, as many Indonesians only use their given names, family members often do not have the same surname.
In all 15 students from Malaysia and Indonesia were detained in the raids on Islamic schools in Karachi over the weekend.
Four more Indonesian students were arrested on Tuesday.
The authorities accuse Mr Gunawan, 27, of running Jemaah Islamiah operations in Pakistan.
"We are interrogating Gunawan to determine if he had any plan to harm Western interest. I think he will remain in our custody for some time," Mr Hayat told the Associated Press.
Correspondents say it is not clear whether Mr Gunawan has any link with his brother's alleged terror network.
Although officials say the student was picked up on Saturday, Karachi's Abu Bakar University where he was studying says he was arrested on 1 September.
Tuesday's arrests had also been made from its campus, the university confirmed, bringing to 11 the number of Indonesian and Malaysian detained there.
The Indonesian authorities say no charges are pending against Mr Gunawan and they are not seeking his extradition.
The Malaysian embassy in Islamabad said it expected its citizens to return to Malaysia after investigations and immigration checks had been carried out in Pakistan.
There are hundreds of foreign students studying in Karachi's Islamic seminaries.
The government has cracked down on them and many foreign students have been forced to leave because they did not have permission from their governments to study in Pakistan.