Bal Thackeray, the leader of another Hindu hard-line group, the Shiv Sena, wrote in June in the group’s weekly magazine that Hindus should defend themselves from Islamist attacks by forming their own squads of suicide bombers.

“The threat of Islamic terror in India is rising,” Mr. Thackeray wrote, according to a translation from the Marathi language that was published in The Hindu, a national English-language daily. “It is time to counter the same with Hindu terror. Hindu suicide squads should be readied to ensure the existence of Hindu society and to protect the nation.”

Prosecutors said that investigators of the Malegaon bombing on Sept. 29 traced a motorcycle at the site of the explosion, apparently used to plant the bomb, to a Hindu nun named Pragya Singh Thakur, 37, who lives nearby in Gujarat State. While in college, Ms. Thakur was a member of the student wing of the Bharatiya Janata Party, according to Vishnu Dutt Sharma, a senior leader of the student wing.

Mr. Sharma described Ms. Thakur as “aggressive in her speeches and work.” She was religious and studious, Mr. Sharma said, and did postgraduate work in history.

During a pilgrimage in 2007, Ms. Thakur renounced a worldly life and became a nun, or a sadhvi in Hindi, cutting her hair short and donning orange robes, the sacred color of Hinduism, according to a brother-in-law, Bhagwan Jha. After she became a nun, her name was changed to Purnachetnanand Giri, which means complete consciousness.

Ms. Thakur’s lawyer, Naveen Chomal, said she had done nothing wrong and that the police had arrested her only because her motorcycle was found at the site of the bombing. The police have said they also have taped telephone conversations in which Ms. Thakur wondered aloud why the Malegaon bombing had not inflicted a higher death toll.

Image Pragya Singh Thakur, shown in 2007, has been arrested in a September bombing. Credit... Associated Press

Some people have begun to treat the suspects as heroic figures. Several Hindu organizations have rallied to Ms. Thakur’s side, contributing to a fund for her legal defense.