Nairobi, Kenya (CNN) -- An additional suspect was arrested Sunday in connection with this month's terrorist attack at a mall in Kenya's capital, the country's interior minister said.
The arrest brings to nine the total number of people in police custody in connection with the attack on Nairobi's Westgate Shopping Mall, Interior Minister Joseph Ole Lenku told reporters.
He declined to release the identities or nationalities of the suspects allegedly involved in the September 21 terror attack, which resulted in a four-day standoff that left at least 67 people dead and parts of the shopping center destroyed.
"We believe it is premature and would jeopardize the results of the investigations if we go giving you the identities of the suspects arrested," Lenku said.
Lenku said authorities have interviewed 156 witnesses so far and found assault weapons in a vehicle they believe was used by terrorists involved in the attack.
On Friday, the Kenya Red Cross said that 59 people remained missing after the attack. And last week outside a city morgue, CNN reporters saw dozens of families who were waiting for word, unsure whether their loved ones were dead or alive.
But Lenku said Kenyan authorities have received no formal reports of missing persons, and believe that there were no hostages in the mall at the time the siege ended. The interior minister asked for aid agencies to forward any information they've received to the government.
"It is the government position that there were no hostages and we managed to rescue all people from the building," Lenku said.
Lenku called for the U.S. government to lift an advisory warning travelers about Kenya, describing the State Department advisory as "uncalled for, unnecessary and unfriendly."
Travel advisories are counterproductive in the fight against terrorism, he said.
"We cannot and shall not allow terrorists to instill fear in our people," he said.
The State Department warning -- first put out in July -- said Americans already there or planning to go there should "evaluate their personal security situation."
Lenku said the list of people killed in the attack includes:
• 57 Kenyans
• 2 French nationals
• 1 Chinese
• 1 Ghanaian
• 1 Canadian
• 1 Korean
• 1 Australian
• 1 South African
• 1 Peruvian
• 1 Dutch national
The Kenyan interior minister did not mention any British nationals, but the British Foreign and Commonwealth Office has said that six British nationals are among the dead. At least three of them had dual citizenship, according to British media reports.
CNN's Victoria Eastwood reported from Nairobi. CNN's Catherine E. Shoichet reported from Atlanta.