Just hours after the MBTA reported a rise in thefts of catalytic converters, undercover MBTA Transit Police detectives yesterday arrested a New Bedford couple who police believe are responsible for as many as 100 of the thefts, Deputy Chief Lewis Best said.

Dennis H. Fager, 39, and Tina M. Fager, 37, were allegedly observed by detectives stealing a catalytic converter from beneath a Toyota Sequoia parked at the Hanson commuter rail lot and placing it in a truck along with two other converters.

Those other catalytic converters had been stolen from cars at the commuter rail lot in nearby Halifax, Tina Fager allegedly told police.

That brought the total number of catalytic converters stolen from MBTA lots this month alone to 26, said Best, head of the Transit Police investigative unit. That followed a year in which 97 were stolen, a fivefold increase over the 18 converters stolen from cars parked at MBTA lots in 2010.

The increase mirrors a national trend, with the converters targeted by thieves as the value of the precious metals they contain has spiked.

The couple, who had been living in a New Bedford motel, are believed to be responsible for most of the catalytic converter thefts from MBTA lots, Best said. Nearly all of the T thefts have occurred at five lots on the South Shore: Hanson, Halifax, Kingston, Lakeville, and East Weymouth.

The Fagers were to be arraigned in Plymouth today on three counts each of larceny over $250 and three counts of malicious destruction of property, while Tina Fager was also slated to be charged with possession of a Class A substance (heroin), Best said.

“It’s a very significant arrest for us,” he said. “It’s been a great nuisance to us, and these thefts have plagued our Southeastern Massachusetts parking lots for some time.”

Transit police investigators worked with local departments to share intelligence and to conduct visible patrols of the lots, while T detectives conducted stakeouts, Best said, with detectives Sean Conway and T.J. Leonard making the arrests. The investigation is ongoing.