Federal drug agents searched the Houston clinic of Michael Jackson’s doctor on Wednesday, according to a news report, even as investigators in California sought more information from the man.

A television channel reported that officials with the Drug Enforcement Administration were at Dr Conrad Murray’s clinic in north Houston.

Video footage showed two Houston police officers guarding the front door.

Murray was Michael Jackson’s personal physician, and was with Jackson when he died.

Murray, who is based in Las Vegas and is licensed in California, Nevada and Texas, has been interviewed by police but he has not been considered a suspect in the singer’s death.

Meanwhile, investigators in California are also seeking more information from Murray, according to his attorney, Edward Chernoff.

The attorney posted a statement on his law firm’s website late on Tuesday saying investigators from the Los Angeles County coroner’s office have asked for medical records in addition to those already provided by Murray.

“The coroner wants to clear up the cause of death; we share that goal”, Chernoff said in his statement.

“Based on Dr Murray’s minute-by-minute and item-by-item description of Michael Jackson’s last days, he should not be a target of criminal charges.”

Murray has emerged as a central figure in the investigation into Jackson’s death.

The doctor, who had been recently hired by Jackson, was with him in his mansion and tried to revive him.

Although Chernoff has said the doctor didn’t give the pop star any drugs that contributed to his death, Murray has nonetheless received attention from those angry over the singer’s death.

Investigators found the powerful anesthetic propofol in Jackson’s home, according to a person with knowledge of the investigation.

The person is not authorised to speak publicly and requested anonymity.

The officials are working with the Drug Enforcement Administration and California attorney general’s office to determine how the medications got there.

Police detectives have already spoken to Murray twice – once immediately after the singer’s death and again two days later.

Police investigators say Murray is cooperating in their investigation.

A cause of death has yet to be determined for the pop star. The coroner’s office is expecting to release autopsy results next week.

Chernoff’s spokeswoman Miranda Sevcik said the interview with coroner’s investigators might happen on Friday.

Murray is currently in Las Vegas, where he has a medical office, she said, and Chernoff would be speaking to investigators without the doctor being present.

Children with stressed-out parents may be more prone to developing asthma associated with environmental triggers such as high levels of traffic-related pollution and tobacco smoke.

Researchers studied 2,497 American children aged 5-9 years old to investigate whether low socioeconomic status or parental stress modified the effect of traffic related pollution and tobacco smoke exposure on the onset of asthma in children. The children were free of asthma or wheezing at the outset. The researchers measured stress in the parents using a standard questionnaire and collected data on air pollution and exposure to tobacco smoke during pregnancy. During the study 120 of the children developed asthma.

It was found that children whose parents reported high levels of psychological stress and who were exposed to cigarette smoke in the womb and to traffic-related pollution early in life had a much higher risk of developing asthma, compared to children exposed only to pollution.

Stress, as well as low levels of education in the parents, was also associated with larger effects of exposure to tobacco smoke during pregnancy.

However, a child exposed to traffic-related pollution whose parents felt their lives were unpredictable, uncontrollable, and overwhelming – suggestive of high levels of stress – had a 51% higher risk of developing asthma during follow up compared with a child exposed to traffic pollution but whose parents had low levels of stress.

The researchers concluded that it was children exposed to the combination of air pollution and life in a stressful environment who were at highest risk of developing asthma. Air pollution can promote inflammation in the airways of the lung, which is a central feature of asthma. Stress may also have pro-inflammatory effects and this may help explain why the two exposures together were important.