Although crime is dropping in Los Angeles, there is one glaring exception: bicycle thefts, which rose 29% last year.

Nearly 2,000 bikes were reported stolen last year -- and authorities believe the actual number of thefts was much higher because so many people don’t report stolen bikes.

Los Angeles Police Department detectives believe the increase is due in part to more people using bikes to get around in some neighborhoods.

A Times analysis of LAPD data found the USC campus area, Venice, parts of Hollywood and downtown L.A. to be hot spots for bike thefts.

LAPD detectives recently broke up a downtown bicycle theft ring and nabbed two men who allegedly swiped bikes downtown and sold them on Craigslist.com. At the motel of one of the alleged thieves, they found bolt cutters, hacksaws and a Mercedes-Benz equipped with a bike rack.

Some bike messengers last month took justice into their own hands when they caught two suspected thieves, teenage boys who attended a local Catholic high school.

According to police, the messengers stripped down the teens to their boxer shorts before taking their cellphones, backpacks and clothes.

“They meted out street justice. We don’t condone street justice. They never threatened them. But they made it clear: Don’t mess with another person’s property,” said LAPD Lt. Paul Vernon. “This incident and the arrests are the tip of the iceberg when it comes to people stealing bicycles.”

Vernon said the two boys told police they were robbed about 3 p.m. Jan. 12 by about 20 men on bicycles at 6th Street and Grand Avenue.

Investigators said they cannot prove the boys were stealing bikes and are continuing to look for the assailants.

Still, the incident has been the talk of the downtown biking world.

“There wasn’t any violence. . . . They were stripped of their clothes and sent home,” said Douglas Forrest, a process server and bicycle messenger who heard about the incident.

In downtown, the number of bikes reported stolen increased last year by more than 57% -- and cyclists have noticed.

“They’ll take anything they can get these days. It has gotten really bad,” Forrest said. “They even go as far as [to] rip off the cranks. People glue ball bearings in the Allen wrench holes under their saddle to protect them.”

Downtown, bicycles are most likely to be stolen between noon and 6 p.m., and Wednesdays are the hottest days for theft, according to an LAPD analysis.

The Richard J. Riordan Central Library, named for the former cycling mayor, is a favorite spot for thieves.

“The library has the worst bicycle racks,” said L.A. cycle activist Stephen Box.

Poorly designed racks out of sight of passersby make the crime easier for bike thieves, he said.

Box said some of worst-positioned racks can be found at the new LAPD headquarters. Some of the upside-down, U-shaped racks the city uses have even been cut and the gap covered with stickers, he said. Bike thieves simply slip out the locked bicycles.

The federal National Crime Victim Survey found that for every bicycle reported stolen, another four such thefts remain unreported. Some transit experts say even that number is a severe underestimation.

Box said many avid cyclists with unique or high-end bicycles know once a bike is stolen how to go into top gear to get it back. They hit Twitter, the guys selling cycles on the corners and the ads on Craigslist. Box said to recover their wheels, cyclists will pretend to be buyers.

According to the LAPD’s Vernon, that is exactly what Central Division detectives did to break the downtown bicycle theft ring.

After a bike was reported stolen Jan. 22 in the 500 block of Olympic Boulevard, a Santa Monica man that same day listed the Sirrus cycle valued at more than $600 for $300 on Craigslist. Undercover detectives posing as buyers met with the suspect, Konstantin Rostovtsev, to acquire the bike, which also included an onboard computer. When the 51-year-old Ukrainian native agreed to the sale, detectives arrested him on suspicion of receiving stolen property.

Investigators used Rostovtsev’s cellphone to identify Edward Rene Arciga, 43, as the man who sold him the cycle. “So we called Arciga and asked if he had any bikes for sale and then our undercover detectives met with him at the Olive Motel on Sunset” Boulevard, Vernon said.

Arciga, a 43-year-old New York native, took the detectives to his aging Mercedes- Benz and pulled out another bike. When detectives arrested him, another man opened the motel room door, revealing several bolt cutters, hacksaws and a few more stolen bikes, Vernon said.

Inside the motel room, Vernon said, officers found bleached small bills that were being turned into counterfeit $100 bills, a computer and evidence of heroin use.

Arciga was arrested on suspicion of receiving stolen property.

richard.winton@latimes.com