EBay's North San Jose campus reopened this morning, clearing the way for 1,900 employees to return to work after a powerful explosive blew out a window Tuesday night and closed the campus all day Wednesday.

The company has increased security at the campus, which includes the headquarters for the PayPal division, spokeswoman Catherine England said. She declined to elaborate on specific security measures.

Bomb-sniffing dogs and investigators spent Wednesday at the site, looking for any signs of who planted the explosive. San Jose Police Sgt. Nick Muyo said the investigation is ongoing.

Nobody was injured in the Halloween night blast -- and other Silicon Valley companies were not put on alert.

The company received no threats before or after the blast, which happened at about 7:30 p.m. Tuesday in a landscaped breezeway outside the 100,000-square-foot, four-story building.

But the corporate target was so sensitive, the explosion so powerful and the bombing so rare in Silicon Valley that law enforcement from San Jose, the federal Bureau of Alcohol, Tobacco, Firearms and Explosives and FBI converged on the campus.

After more than 24 hours scouring for clues, investigators had few answers. Was it someone intentionally targeting the Internet corporate giant, a techno-terrorist? Or was it a dangerous prank on a night known for smashed pumpkins and toilet paper attacks?

"It could have been a Halloween thing, it could have been kids, it could have been something larger," Muyo said. "If it's a Halloween prank, it's a sophisticated one."

Bomb-sniffing dogs spent the day nosing around the office and other campus buildings, all emptied of its employees, and at eBay's headquarters on Hamilton Avenue. They found nothing suspicious.

And investigators said there was no indication that the company had been warned or that the bomber had targeted any other place. No one had taken responsibility for the blast as of Wednesday night.

EBay officials said eBay and PayPal service was not disrupted. Many of the processes that keep the Web pages running can be handled from other locations, eBay spokesman Hani Durzy said.

"The safety and security of our employees is our top priority," Durzy said.

Muyo described the source of the blast as a sophisticated explosive device that not only shattered a large, two-inch-thick plate of safety glass, but also blew over furniture inside.

"Whatever caused it was pretty strong," San Jose fire Capt. Jose Guerrero said.

About 1,900 eBay employees work at the North San Jose campus, which includes PayPal headquarters, technology staff, sales, marketing and the network operations center, Durzy said. Employees at that campus were asked to work at the company's other San Jose location or dial in from home, he said.

When PayPal employee Dan Hyder arrived for work Wednesday morning, he was told the office was closed and given a flier explaining what had happened. He walked around the site for a while afterward, taking photos of the investigators milling around the building where he used to work.

"It's bizarre," said Hyder, who had already left work before the blast Tuesday night.

Muyo said the timing of the explosion, at 7:34 p.m. when many people had already left work for the day, might indicate that whoever placed the device at the building wasn't looking to injure people.

Authorities received multiple reports of a smoke detector sounding off, Guerrero said, followed a few minutes later by multiple calls of an explosion.

When crews arrived, they found a 6-by-7-foot window had been shattered near a first-floor exit. The window's frame was bent and a light haze covered the area, Guerrero said. No flames, however, were visible, nor was there any other damage. Other debris was in the area, said Guerrero, who couldn't say exactly what the debris was.

Authorities have an idea about what may have caused the blast, Guerrero said, but he declined to disclose it. Muyo declined to say whether security cameras may have captured images from the site.

"We're not ruling anything out," Muyo said. Neither PayPal nor its corporate parent eBay received any threats before the blast, Muyo said.

"It was fortunate that no one was hurt," said Catherine England, an eBay spokeswoman.

From Google to Yahoo, other Silicon Valley corporations reported no similar threats or incidents.

Security experts said such dangers unfortunately come with the corporate territory.

"You're a target if you have a commodity that's of any particular value, whether it's home invasion or the CEO being kidnapped. It doesn't have to be a terrorist group; it could be the Animal Liberation Front," said Forrest Franklin, director of operations for Focus Group Consultants in Carson City, Nev., who has worked as a consultant for Silicon Valley companies. "If you've got 3,000 people in your company, you've got 3,000 potential perps."

Ken Silva, chief security officer for VeriSign, agreed that such attacks can come from disgruntled employees.

"It could also just be a random person who is disgruntled with the company," Silva said. "It doesn't take much among millions of people out there for someone to get angry."