The San Francisco Zoo reopened under heightened security on Thursday, nine days after an escaped tiger killed a 17-year-old boy and injured two of his friends.

The zoo has been equipped with a new public-alert system and more signs urging visitors not to pester the animals, zoo officials said.

The pen housing tigers and lions remains closed indefinitely while architects design a new, more secure enclosure that will include a six-metre wall separating the cats from visitors.

Zoo officials have said the pen's current four-metre wall is about a metre short of the recommended minimum height for American zoos.

Witness says victims taunted animals

Police and zoo officials are investigating whether the victims taunted the 160-kilogram Siberian tiger before it escaped from its outdoor pen and killed Carlos Sousa Jr. Sousa's friends — brothers Paul Dhaliwal, 19, and Kulbir Dhaliwal, 23 — were injured.

One witness told the San Francisco Chronicle that she saw a group of young men roaring at the lions in the same enclosure shortly before the tiger's rampage. She said Sousa was in the group of young men but did not participate in the taunting.

"They were trying to get [one] lion's attention…The lion was bristling," Jennifer Miller told the Chronicle.

Police can't confirm taunting report

San Francisco police said they have been unable to corroborate reports that the victims taunted the tigers. Taunting an animal at a zoo is a misdemeanor.

"I don't know if what they did was any more than what kindergartners do at the zoo every day," Insp. Valerie Matthews told the Chronicle.

Zoo officials will not comment on whether they suspect that taunting caused the tiger to leap out of its cage.

"All I know is that something happened to provoke that tiger to leap out of her exhibit," zoo director Manuel Mollinedo said during a press conference Wednesday.

'Calculated attack on these victims'

Mark Geragos, a lawyer representing the injured brothers, denied his clients teased the animals and accused the zoo administration of "peddling unfounded rumors."

"It's unconscionable," he told the Chronicle. "They're doing nothing but a calculated attack on these victims ... when in actuality the zoo security didn't do what they should have been doing after the attack."

He maintains that the brothers sought help after the attack, but zoo employees didn't respond for 30 minutes.

With files from the Associated Press