Meanwhile, insurgents wearing suicide vests tried to storm NATO's largest base in the south, but did not breach its defenses, officials said.
Alhaj Zaher Vahdat, deputy governor of Balkh province, said one person has been arrested in connection with the robbery Monday night at a branch of Kabul Bank in the northern city of Mazar-e-Sharif.
Sherjan Durani, a police spokesman for Balkh province, said someone apparently mixed the poison into the guards' food. The guards showed evidence of having vomited, he said. The robbers used small and large knives to stab the guards in the neck, stomach and heart, then fled with about $269,000 in U.S. and Afghan currency. Earlier reports that the guards had been beheaded are inaccurate, Durani said.
Kabul Bank officials declined to comment on the incident until it has been investigated.
The assault on Kandahar Air Field started just before midday Tuesday with two rockets fired into the base, then a handful of would-be suicide bombers assaulted the outer gates, said Commander Amanda Peterseim, a spokeswoman for NATO forces.
She said there had been no reports of NATO casualties, and that all of the attackers were killed in the fighting. Previous reports that some of the attackers had fled were incorrect, she said. Peterseim said there were "approximately four" people in suicide vests. She said none of the assailants at the gates survived.
"They either detonated themselves or were killed," Peterseim said.
The Taliban claimed responsibility for the attack, saying they sent seven suicide bombers to assault the base. At least one of the bombers detonated his explosives at a base checkpoint, said Qari Yousef Ahmadi, a spokesman for the insurgent group.
Kandahar provincial government spokesman Zalmai Ayubi said two civilians who were outside the base were injured in the fighting.
Taliban insurgents previously tried to storm the Kandahar base on May 22, just a few days after the insurgents sent suicide bombers to assault the main U.S. base in the country - Bagram Air Field near the capital.
Also in the south, three private security guards were killed and four others injured Tuesday by a roadside bomb in Arghandab district of Zabul province, according to Amanullah, provincial chief of police who goes by one name.
Associated Press Writers Amir Shah and Heidi Vogt in Kabul and Mirwais Khan in Kandahar contributed to this report.