A police investigation into a Brazilian nightclub fire that killed 231 people and left dozens in critical condition turned Tuesday to whether key evidence has been hidden.

Authorities have taken four suspects into custody following the tragedy early Sunday at the Kiss discotheque in the southern college town of Santa Maria, described as the second worst inferno in the country's history.

Those detained Monday include two owners of the club and a pair of musicians who allegedly took part in an ill-fated pyrotechnic show being blamed for sparking the blaze.

But with many questions still unanswered, police on Tuesday were probing whether security cameras had recorded what happened and whether the place -- packed with college students -- was overcrowded.

"The video cameras weren't in the nightclub (where they were supposed to be) and there was no computer with memory for storage," prosecutor Veruska Agostine told reporters.

Some survivors claimed the vocalist of the Gurizada Fandangueira band lit a firework that may have triggered the blaze, which broke out at around 2:00 am (0400 GMT) Sunday. Police were trying to pinpoint who brought the fireworks to the club and activated them.

On Monday, police chief Marcelo Arigony told reporters: "The place may have burned down because of the firework and because the door was not wide enough for people to leave."

The club had only one exit and was blocked by steel barriers that made it difficult to get out. Some 180 bodies were found in the bathrooms, which many panicked party-goers had apparently confused with emergency exits.

Most of the victims died of smoke inhalation in their desperate bid to escape.

"We are investigating to be sure there were no (security camera) images," Arigony said. The owners of the club, meanwhile, contend the surveillance system had not been working for two months.

According to local media, the club's cash register, which would help determine whether the establishment was overcrowded, was not to be found.

In other developments, the state of Rio Grande do Sul where Santa Maria is located, sought to freeze the assets of both the club owners and their company.

"The aim is to ensure the right of people to guaranteed compensation," said the state's attorney general, Nilton Arnecke Maria.

Late Monday, thousands of people marched in silence through Santa Maria, demanding justice for the dead.

"I know that my daughter isn't coming back. But somebody has to pay for this," said marcher Jorge Neves, who lost his daughter Rafaella.

Meanwhile, mayors of several cities announced more stringent oversight of nightclubs during their annual meeting with President Dilma Rousseff, who urged them to ensure that such a "terrible tragedy never takes place again."

The massive South American nation is observing three days of mourning as distraught families and friends continue to bid farewell to the victims as others clung to life. As of Monday, 75 survivors were in intensive care.