Sonoma County Sheriff's detectives are on the trail of at least 13 suspects they think are responsible for stealing checks, altering them and cashing them for large sums of money.

They're tracking at least 21 cases in which checks have been stolen from mail boxes, altered or counterfeited, and cashed at local banks. Two suspects had been arrested, but others are sought.

"This (mail theft) is not something new, but we've seen a big spike in it all of a sudden" said Sgt. Mike Raasch, who leads the sheriff's property crimes investigation unit. He said he believes the cases his and other law enforcement agencies have tracked since September may be connected, possibly through an organized check-cashing ring.

As he spoke, the wall behind his desk was covered with mug shots of suspects, who mainly are from East Bay cities. A particularly clear trail of clues has been left, he said. Sometimes the culprits counterfeit new checks but more often they wash the original ink from stolen checks and then write the check out again, usually for a certain amount: $923.17. More recently, some checks in the amount of $1,200 have been detected. The counterfeiters often leave another clue on the check's memo line: a five-digit number, followed by a dash and three more numbers, he said.

The check is then taken to the bank where it originated and cashed, often by people using their real identification, Raasch said.

Mendocino County investigators also noted an increase in mail theft and issued a press release to warn residents.

"For the past several weeks unknown persons have been stealing mail from roadside mailboxes primarily in the Ukiah and Redwood Valley areas," said Mendocino Sheriff's Capt. Greg Van Patten in the statement. He said the thefts were happening primarily during the late night and early morning hours. He asked that anyone who sees someone tampering with a roadside mailbox to call 463-4086.

The Sonoma County detectives are collaborating with U.S. Postal Service inspectors as well as Rohnert Park, Santa Rosa and Sebastopol police as they pursue the case. They're also in touch with other jurisdictions, including the Monterey County Sheriff's Office and Carmel police, that are tracking some of the same suspects.

This week, cases continued to arrive on Raasch's desk. Reports seem to be picking up, he said. There have been at least seven since Jan. 1. Still, Raasch estimates that about half the crimes have gone unreported, as people often don't realize right away that mail has been stolen.

"The banks are now aware that there's a trend and they're scrutinizing checks a lot more carefully," he said. No bank interviewed by the Press Democrat said it had experienced a major increase in check fraud in recent months, but that could be because the fraud has been spread out across numerous institutions. Fraudulent transaction and mail thefts in Sonoma County have been detected in Santa Rosa, Sonoma, Rohnert Park, Windsor, Cotati and Sebastopol, Raasch said.

Bank officials said that they are working with police and alerting their staff to watch out for fraud. They also emphasized that account holders are not financially responsible for fraudulent checks cashed -#8212; the banks are.

Brad Hunter, senior vice president at Exchange Bank, said that his bank has alerted its tellers to the current problem.

"The main thing is just to know your customer," he said. "It's usually hard for these guys to get through at a community bank."

On Jan. 17, an alert Sebastopol teller at Exchange Bank noticed the signs of fraud, leading to an arrest.

The teller, who was aware of the scam, became suspicious of a woman who tried to cash a check for $923.17, Raasch said. She stalled the woman while the Sebastopol police were called. They arrested Tammy Marie Forse, 36 of Berkeley. On Friday, she pleaded no contest to one count of burglary. A count alleging forgery was dismissed.

A second woman, for whom the Sheriff's office in December obtained a $140,000 arrest warrant, was apprehended on separate charges in San Jose early this month, Raasch said. Nathalie Lopez-Gutierrez, 18, of San Jose currently is being held in Santa Clara County on a charge of assault with a deadly weapon, said a spokesman with the Santa Clara County Superior Court.

Jeff Fitch, a postal inspector with the U.S. Postal Service, said the agency has a standing $10,000 award for information that leads to an arrest and conviction for mail theft. The federal penalty is up to five years in federal prison and a fine of up to $250,000, he said.

Fitch emphasized the importance of reporting mail thefts, both to the Postal Service and local law enforcement agencies. That can done by going to usps.com and clicking on the postal inspectors link at the bottom of the page.

Raasch and Fitch recommended that people bring checks or other items of value directly to the post office or deposit them in a blue collection box. If putting outgoing mail into a personal mail box, it's best to do so as close as possible to when the mail carrier comes by, they said.

A locking mail box is a good precaution, Raasch said, noting that once information is taken, a person can be a victim of identity theft for years.

"I know it's inconvenient, but it's a sign of the times," he said. "It's best to go to the post office or hand your mail directly to your mail man."