(CNN) -- A Swedish SWAT team backed by local police arrested four people overnight on suspicion of plotting terror attacks, the Swedish Security Service told CNN Sunday. 

The four were held in the city of Gothenburg for "probable cause" of preparing the attacks, the highest level of suspicion in Sweden, said Sara Kvarnstrom, a spokeswoman for the security force. 

She refused to say whether the suspects had been under surveillance or if the arrests resulted from a tip-off, saying she could not discussed details on an ongoing investigation. 

"Through these arrests we have been able to prevent a situation from occurring," Malena Rembe, head analyst at the Swedish Security Service, told CNN affiliate TV4. 

Sweden is not raising its terror threat level -- which is currently at 3, with 5 as the highest -- and there is "no reason for the public to be alarmed," she said. 

"These arrests have not changed this threat level," Kvarnstrom added. 

An art gallery in central Gothenburg was evacuated shortly before midnight local time, police said, but declined to say whether it was connected to the arrests. 

A party was going on at the Roda Sten gallery at the time, with about 500 people attending the inauguration of an arts festival, according an interview with a witness in Goteborgs Posten, a local newspaper. 

The police cordon was removed at 6 a.m. local time, said Ulf Edberg, a spokesman for Gothenburg police. 

Police have a "number of extra police officers" on the streets of Gothenburg, Edberg said.  QUESTIONS:

Q1:
1. How many people got taken into custody?
2. What was the number of people detained?
3. What was the number of people taken into custody?
Q2:
1. Who carried out the arrests?
2. Who were people detained by?
3. Who took the suspects into custody?
Q3:
1. Who helped the Swedish SWAT team?
2. Who did the Swedish SWAT team receive aid from?
3. Who arrested the suspects along with the Swedish SWAT team?
Q4:
1. What was the reason for the arrest?
2. What had the suspects allegedly done?
3. Why did Swedish SWAT carry out the arrests?
Q5:
1. What organization was the spokesperson for the police?
2. What organization spoke on behalf of the police?
3. Who had something to say as a representative of the police?
Q6:
1. Who was the spokesperson for the Swedish Security Service?
2. Who spoke on behalf of the Swedish Security Service?
3. Who gave comments for the Swedish Security Service?
Q7:
1. What is Sweden's current concern threshold?
2. What number is Sweden's concern threshold?
3. Where does Sweden's concern threshold currently stand?
Q8:
1. Do Swedish citizens need to worry?
2. Should citizens of Sweden feel anxious?
3. Is there reason for citizens of Sweden to be anxious?
Q9:
1. What location were citizens escorted out of?
2. What place did citizens have to leave?
3. Where were citizens ushered out of?
Q10:
1. When were citizens cleared from the art gallery?
2. At what point was the art gallery cleared of citizens?
3. When were people ushered out of the art gallery?
Q11:
1. Why were the citizens in the art gallery?
2. What had brought citizens to the art gallery?
3. What was the citizens' reason for being in the art gallery?
