Kansas City, Mo. ? Six gazelles were killed when a pack of stray dogs made its way into the Kansas City Zoo.

The attack early Thursday was the second of its kind in as many years. The zoo was ordered to improve its security fencing in July 2002 after stray dogs killed three sheep and injured two other sheep and an emu.

The zoo learned of the latest attack after keepers spotted the strays in the gazelle holding area when they arrived for work around 8:30 a.m., said Randy Wisthoff, zoo director.

Keepers chased the dogs, which ran onto the Africa Plains area. Fearful that the dogs would attack other animals, two of the four strays were shot to death and a third dog was caught in a net before the zoo opened. A fourth dog was not immediately found. None of the animals had collars.

“It’s an unfortunate thing,” Wisthoff said. “We have done a walk of the entire zoo, and we still haven’t been able to figure out where the dogs came in.”

The eight slender-horned gazelles that survived the attack will be housed indoors until zoo staff can guarantee their habitat is secure. An initial search of the fencing revealed no obvious breaches, Wisthoff said.

The zoo will report the attack to the U.S. Department of Agriculture, which checks the fences during regular inspections.

Strays pose problems for zoos, which are stocked with tempting prey. Authorities have blamed packs for killing animals elsewhere, including more than a dozen wallabies in 2002 at a zoo in South Bend, Ind.