The condition of American missionary Joseph W Cooper (60), who was injured in an attack by suspected Rashtriya Swayamsevak Sangh activists at Kilimanur near Thiruvananthapuram, was stated to be stable on Wednesday.
Benson K Sam, the local preacher who also suffered minor injuries, said Cooper's ECG report was normal.
Cooper continued to be in the intensive care unit at the Kerala Institute of Medical Sciences hospital in Thiruvananthapuram.
Kilimanur police sources said an RSS worker, Raju, was arrested and four others were taken into custody for interrogation.
A case had been registered under section 307 of the Indian Penal Code IPC for attempt to murder and also under the Explosives Act and for rioting, they said, adding the police was on the look-out for two others.
"This is a very serious matter. You cannot just go on attacking anybody," a US embassy spokesperson was quoted as saying in New Delhi. "Naturally we are deeply concerned."
The embassy said it had contacted its Chennai consulate for details of the attack.
Benson said US officials at Chennai had enquired about the condition of the missionary.
Various political leaders, condemning the incident, held the Sangh Parivar responsible for the attack.
Chief Minister A K Antony termed the incident as "unfortunate".
Cooper and a group of evangelists were waylaid and attacked on Monday night while they were returning after attending a Bible convention at Koppam near Kilimanur.
Communist Party of India leaders J Chitharanjan and C Divakaran, who visited Cooper, asked the government to take steps to check recurrence of such incidents.
RSS leader K Rajasekharan, meanwhile, said the Sangh workers were being falsely implicated in the case.
He said the whole incident was "shrouded in mystery".
"It was unlawful on the part of Cooper, who was on a visiting visa, to preach," he added.
