(CNN) -- A Houston police officer was wounded Thursday as a myriad of agencies took part in an operation that has resulted in the arrests of about 100 Mexican cartel cell members in cities across the United States, officials said.
The officer was shot as he tried to serve a narcotics warrant as part of the sweep, Drug Enforcement Administration spokesman David Levey told CNN.
The operation, which began Wednesday, is a joint effort by the DEA, Immigration and Customs Enforcement, the Bureau of Alcohol, Tobacco, Firearms and Explosives, and other state and local agencies.
As of Thursday morning, more than $4 million had been seized, and about 100 people had been arrested, DEA spokesman Michael Sanders told CNN.
In Houston, Nash Patel, 39, was one of several police officers who went to serve a warrant at a home. When there was no answer at the door, the officers forced their way in, and in the process of clearing the house found a man in the back armed with a pistol, police spokesman Kese Smith said.
The man looked at the officers, who were wearing police vests, and opened fire, Smith said. Officers returned fire, striking the suspect. The suspect's condition was unknown, he said.
Patel was struck in the elbow and lower backside but was in good condition at the hospital, Smith said.
The sweep comes a week and a half after an ICE agent working in Mexico was ambushed on a highway, but the operation was not directly in retaliation for that killing, said Sanders, the DEA spokesman.
"We're looking at it as going out there and doing these investigations, not as a retaliation, but to say, 'Look, we are going to investigate you,' " he said.
The DEA's Levey said that because the operation was targeting Mexican drug cartels, it was definitely a response to the killing of agent Jaime Zapata in Mexico.
The agencies are trying to send a message to cartel operatives in the United States that they cannot get away with illegal activities, Sanders said.
He described most of the targets as mid- to high-level dealers, the type who have day jobs but work in cells that distribute drugs and return drug money to Mexico.
The people arrested this week belong to several cartels, Sanders said. The cells are small and work in an isolated fashion, so it's possible that multiple cells from the same cartel may operate in the same city without knowing each other.
The sweep was not unlike other joint operations against drug traffickers, Sanders said.
Those arrested could face federal drug charges or various state charges, depending on the evidence collected.