Bolivian journalist's family wants to know who was behind attack.

(CNN) People march in Yacuiba in southern Bolivia,  to demonstrate against the attack on Fernando Vidal.

 Just days after an arson attack left a Bolivian radio personality in critical condition, police arrested four suspects.

Now, the victim's family says, comes the real test: Will investigators uncover the mastermind behind the attack?

The daylight attack on Radio Popular, a small FM station in the southern city of Yacuiba, happened Monday, as owner and host Fernando Vidal was on air conducting an interview.

Four men forced their way into the radio station, said Vidal's son-in-law, Esteban Farfan. Two entered the studio.

Listeners heard live on the radio the gunshot of one of the men, and the ensuing scuffle between Vidal and one of the intruders, while the other poured gasoline on him and his equipment. They lit Vidal on fire and fled.

The 70-year-old radio personality, a former mayor and councilman, survived, but remains in critical condition with burns to his face and body, Farfan said.

Another employee in the room, Karen Arze, suffered burns to a lesser extent and also is hospitalized, he said.

Others escaped the radio station through the windows.

The assault sparked international condemnation. It was a "vicious and brazen" attack, the spokesman for the U.N. High Commissioner for Human Rights said.

Three men were arrested the next day as they tried to cross the border into Argentina, just miles from Yacuiba. They were charged preliminarily in connection with the attack, police commander Orlando Gongora said. A fourth suspect was arrested Wednesday and is awaiting a formal preliminary charge, he said. Police worked for 48 hours straight to identify the attackers, Gongora said, But Vidal's family insists that the incident has deeper roots.

"(The police) have done the easiest thing -- to arrest the four suspected attackers," Farfan said. "Now we need to reveal the masterminds behind this."Vidal is an outspoken and respected journalist who made enemies with his opinions, Farfan said.

A leftist, Vidal was once close to President Evo Morales, but the journalist broke off his support over what he described as the president's turn to authoritarianism. He is a relentless critic of corruption, Farfan said. He calls out politicians who allegedly grow their personal bank accounts with state funds and routinely calls for investigations into corruption.

At the time of the attack, Vidal was interviewing two guests on the topic of smuggling in the border area, leading to initial speculation that an organized crime group tied to the smuggling was behind the incident.

But that explanation seems too convenient to Farfan. He believes that political opponents had a role in the attack against his father-in-law. "The message is clear," Farfan said. "(The attackers) want to shut us up."

But the cash-strapped station borrowed equipment and was back on the air less than 24 hours after the attack.

The police commander is aware of the theories that someone ordered the attack on Vidal, but said it's too early in the investigation to know for sure.

For police, the motive and any affiliation the four suspects may have with organized crime or other interests remains unknown.

Before their arrests this week, the suspects had been in trouble with the law before, but only for minor robbery charges, the commander said.

That doesn't make sense to Farfan.

"The question is: Who would benefit from Fernando Vidal's commentaries being silenced?"