HAYWARD — From about 1 to 5 a.m. on March 23 of last year, undercover federal agents parked outside Glenio Silva's pizzeria and took notes.

They watched as delivery crews and cooks entered and exited The Pizza House, a tiny fast-food restaurant in Hayward's historic downtown. The agents returned in April and May, staying long enough each trip to watch the business close at 3 a.m., and to confirm that some of the workers never left because they lived in the building.

Tipped off by an informant, an illegal immigrant who quit the restaurant in 2006 after a feud, the agents with Immigration and Customs Enforcement explained in a request for a search warrant that they were building a case against a business they had reason to believe was criminally violating the nation's immigration laws.

On June 15, 2007, the agents came back in full force, raiding the Hayward establishment and a sister shop in San Francisco on a busy Friday night, arresting six unauthorized workers from Brazil and charging Silva, the business owner, with concealing and harboring illegal immigrants — a federal felony.

ICE officials said the case fits into the agency's strategy of increasingly relying on criminal prosecutions against employers to enforce immigration law. Yet a review of Bay Area immigration prosecutions indicates that such prosecutions remain a rarity, and they are hardly routine. In the first six months of the current fiscal year, only three people were convicted of concealing and harboring illegal immigrants in federal court in California's Northern District, which covers the Bay Area and all of coastal California from Monterey to the Oregon border, according to data culled from the nonprofit Transactional Records Access Clearinghouse.

"I don't think the laws are really in sync with reality," said Silva, who is now contesting his criminal charges in a San Francisco court. "All the restaurants I know — everybody with a busy restaurant — has illegal people. I don't think this is fair."

His lawyer, Steven Gruel, a former federal prosecutor, calls Silva's situation a case of "unconstitutionally arbitrary enforcement" — and is using ICE's haphazard enforcement record as one reason the case should be dismissed.

"ICE's enforcement of worksite violations is bewildering," Gruel wrote in a motion to dismiss the case, filed last week. "Major violators receive a slap on the wrist or no action whatsoever, undocumented aliens are openly harbored as they seek refuge in sanctuary cities, and yet a lone pizzeria owner with six undocumented workers is caught up in the whirlpool of this federal criminal prosecution."

The charge Silva faces is one of the key tools ICE can use to criminally charge unscrupulous employers, and comes with a penalty of as many as five years of prison and deportation, even though Silva is a lawful permanent resident. In a written statement last month, the agency said that its strategy "differs dramatically from the approach of the former Immigration and Naturalization Service (INS), which focused on imposing civil fines on employers who hired illegal aliens."

Being in the country illegally is still an administrative violation, not a crime. But there are several related crimes, ranging from fraudulent use of visas to conspiracy to bring illegal aliens into the country. And in contrast to its predecessor, INS, which was disbanded in 2003 when ICE was formed, ICE officials say their new agency "relies heavily on criminal prosecutions and the seizure of company assets to gain compliance from businesses that violate the employment provisions of our nation's immigration laws."

The total number of immigration crime prosecutions nationwide has risen sharply this year, mostly because of Operation Streamline, a new effort in border states to charge those who cross the border with low-level crimes. But prosecution rates in other parts of the country have not changed as much.

The records-gathering organization reported that 75 people were convicted of immigration crimes in the Northern District of California during the first six months of this fiscal year, the vast majority for the charge of re-entering the United States after being deported — a charge that is frequently attached to cases against people charged with other crimes.

The total number of prosecutions and convictions on immigration charges in the region has remained relatively steady throughout the Bush administration. The highest year for immigration convictions in the region was from Oct. 1, 2000 through September 2001, following a surge in prosecutions issued during the last year of the Clinton administration.

Defenders of current methods to battle illegal immigration say the local prosecution numbers don't reflect the wide array of approaches used to deal with the issue.

"Rather than going after the garden-variety employer who is hiring illegals, the criminal emphasis is on those who are exploiting them," said Joseph Russoniello, the U.S. attorney in charge of the San Francisco region, explaining why the number of employer convictions is so low.

Russoniello said prosecutors focus on the most egregious cases, while ICE is responsible for dealing with employers who should face fines rather than criminal charges.

"We could probably fill up the docket with employment cases," Russoniello said. "It wouldn't do us a lot of good. It would bleed off a lot of resources."

Minimum fines against employers increased this year from $275 to $375 per illegal employee, while the maximum penalty for one illegal employee increased from $11,000 to $16,000. In fiscal year 2007, ICE said it secured more than $30 million in criminal and civil fines from worksite enforcement cases nationwide, arrested 863 people on criminal charges and made more than 4,000 administrative arrests in those worksite cases.

In fiscal year 2008, ICE worksite enforcement teams have arrested more than 2,900 people on administrative violations during the first half of the year, and arrested 850 people so far on criminal charges. Of those people, 75 are in the "supervisory chain" of owners, supervisors and managers, the agency said.

But critics of ICE's methods say the government has targeted more and more undocumented immigrants with costly detention and deportation in recent years while leaving employers who hire them off the hook.

"When you start realizing where all of their resources are going to, it's really about picking up undocumented immigrants," said Evelyn Sanchez, director of the Bay Area Immigrant Rights Coalition. "They kind of see it as a badge of honor that on a yearly basis, they detain almost 1 million immigrants."

Sanchez is providing legal assistance for some of the 63 illegal workers, most from Mexico, who were detained by ICE on May 2 when the agency raided El Balazo, a chain of taquerias across the East Bay and in San Francisco. The owners of the restaurant chain have not faced any criminal charges, but ICE said the business is still under investigation.

Small restaurant chains have been one of the main targets of the most high-profile immigration raids in California. In May, ICE raided a French bakery in San Diego and arrested 15 suspected illegal immigrants there. The bakery owners have not been charged with crimes.

Silva, who sold his family's San Ramon house last year because of the financial losses he has faced following the raid on his two pizzerias, is fighting the allegations that he knowingly hired illegal Brazilian immigrants.

But the part of the situation that causes him to bristle most is that some of the Brazilians who were not authorized to work for him had previously worked at much bigger pizza chains. Why, he asked, hasn't the government gone after those chains?

Mark Krikorian, director of the Center for Immigration Studies, which advocates for stricter enforcement, said he has little sympathy for such arguments, and said that a combination of increased enforcement activity and the poor economy has already caused many illegal immigrants to leave.

"Raids are important," Krikorian said. "The importance of enforcement is not to arrest every illegal immigrant. But the point is to send a message, to both workers and employers, that the party is over."

Oct. 5, 2005 -- Hayward
What happened: ICE raided the Pacific American Service warehouse and distribution center, which imports and exports products from the Port of Oakland. Arrested were illegal employees from Mexico, who ICE said represented at least 25 percent of the company's more than 60 employees
Today: Owners have not been prosecuted.
June 15, 2007 -- Hayward
What happened: ICE raided The Pizza House in Hayward and Monterey Pizza in San Francisco, arresting six illegal workers from Brazil, one a recent graduate of Pinole Valley High School. ICE also charged the owner with a federal crime of harboring illegal aliens.
Today: Owner Glenio Silva is contesting charges that he knowingly harbored illegal workforce, saying allegations by secret informant are false and based on "unconstitutionally arbitrary prosecution."
Nov. 7, 2007 -- Oakland
What happened: ICE raided Pepe's Cabinets, a small furniture-maker's shop in Oakland, arresting eight illegal workers from Mexico and charging the shop owner with criminally harboring them. The 16-month investigation began after someone called the ICE tip line.
Today: Owner Jose De Jesus Guzman-Baez is being prosecuted.
May 2, 2008 -- East Bay, San Francisco
What happened: ICE raided a taqueria chain, El Balazo, arresting 63 illegal workers of the 11-restaurant chain.
Today: Owners have not been prosecuted. ICE says investigation is ongoing.